LoadAd operations return LoadAdResults (#682)

LoadAd operations on BannerView and Interstitial Ads now return a LoadAdResult.  On error this contains information from the AdMob service as to why a failure occurred.  The C++ SDK also uses it to return errors that may have occurred when attempting to marshal the Android and iOS SDKs.

The implementation obfuscates Objective C pointers and Android references via Internal structs. These are forward declared in our public API and defined internally.  These structs are passed to AdapterResponseInfo, AdResult, LoadAdResult and ResponseInfo class constructors.

Other changes include:
 - Updated the use of Futures in BannerView and InterstitialAd to use SafeFutures.
 - Removed the use GetLastResult to construct future objects, and its use opens up the potential issue of race conditions. Replaced this implementation with MakeFuture.
 - Moved LoadAd error code mappings from Android and iOS to C++ code.
 - Updated integration tests to include the following scenarios for both Interstitial and Banner Ads
   - Duplicate initialization invocations on the same object.
   - Duplicate loadAd requests on the same object.
   - The use of an errant AdUnitId string
 - Reenabled a multithreaded deletion test that was previously disabled due to a now obsolete Android bug
 - Added a LoadAd integration test that doesn't require user interaction, so that we can run it on FTL.
 - Updated iOS Admob SDK to 8+, and reworked the InterstitialDelegates and Loading process to match the new 8.0 flow.

[Integration Test run](https://github.com/firebase/firebase-cpp-sdk/actions/runs/1408627388)

Author: DellaBitta

admob/CMakeLists.txt

@@ -34,23 +34,35 @@ binary_to_array("admob_resources"
 set(android_SRCS
     ${admob_resources_source}
     src/android/ad_request_converter.cc
+    src/android/ad_result_android.cc
+    src/android/adapter_response_info_android.cc
     src/android/admob_android.cc
     src/android/banner_view_internal_android.cc
-    src/android/interstitial_ad_internal_android.cc)
+    src/android/interstitial_ad_internal_android.cc
+    src/android/load_ad_result_android.cc
+    src/android/response_info_android.cc)
 
 # Source files used by the iOS implementation.
 set(ios_SRCS
     src/ios/FADAdSize.mm
     src/ios/FADBannerView.mm
     src/ios/FADInterstitialDelegate.mm
     src/ios/FADRequest.mm
+    src/ios/ad_result_ios.mm
+    src/ios/adapter_response_info_ios.mm
     src/ios/admob_ios.mm
     src/ios/banner_view_internal_ios.mm
-    src/ios/interstitial_ad_internal_ios.mm)
+    src/ios/interstitial_ad_internal_ios.mm
+    src/ios/load_ad_result_ios.mm
+    src/ios/response_info_ios.mm)
 
 # Source files used by the stub implementation.
 set(stub_SRCS
-    src/stub/admob_stub.cc)
+    src/stub/ad_result_stub.cc
+    src/stub/adapter_response_info_stub.cc
+    src/stub/admob_stub.cc
+    src/stub/load_ad_result_stub.cc
+    src/stub/response_info_stub.cc)
 
 if(ANDROID)
   set(admob_platform_SRCS

admob/integration_test/Podfile

@@ -4,7 +4,7 @@ platform :ios, '10.0'
 
 target 'integration_test' do
   pod 'Firebase/Analytics', '8.5.0'
-  pod 'Google-Mobile-Ads-SDK', '7.69.0-cppsdk'
+  pod 'Google-Mobile-Ads-SDK', '8.12.0'
 end
 
 post_install do |installer|

admob/integration_test/integration_test.xcodeproj/project.pbxproj

@@ -158,6 +158,7 @@
 			developmentRegion = English;
 			hasScannedForEncodings = 0;
 			knownRegions = (
+				English,
 				en,
 			);
 			mainGroup = 529226C91C85F68000C89379;
@@ -289,6 +290,7 @@
 				CODE_SIGN_IDENTITY = "iPhone Developer";
 				CODE_SIGN_STYLE = Automatic;
 				DEVELOPMENT_TEAM = "";
+				EXCLUDED_ARCHS = i386;
 				FRAMEWORK_SEARCH_PATHS = (
 					"$(inherited)",
 					"$(PROJECT_DIR)",
@@ -303,6 +305,7 @@
 					"\"$(SRCROOT)/external/googletest/src/googlemock\"",
 				);
 				INFOPLIST_FILE = "$(SRCROOT)/Info.plist";
+				OTHER_LDFLAGS = "$(inherited)";
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				PROVISIONING_PROFILE_SPECIFIER = "";
 				WRAPPER_EXTENSION = app;
@@ -317,6 +320,7 @@
 				CODE_SIGN_IDENTITY = "iPhone Developer";
 				CODE_SIGN_STYLE = Automatic;
 				DEVELOPMENT_TEAM = "";
+				EXCLUDED_ARCHS = i386;
 				FRAMEWORK_SEARCH_PATHS = (
 					"$(inherited)",
 					"$(PROJECT_DIR)",
@@ -331,6 +335,7 @@
 					"\"$(SRCROOT)/external/googletest/src/googlemock\"",
 				);
 				INFOPLIST_FILE = "$(SRCROOT)/Info.plist";
+				OTHER_LDFLAGS = "$(inherited)";
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				PROVISIONING_PROFILE_SPECIFIER = "";
 				WRAPPER_EXTENSION = app;