Fix dexing. (#2612)

* Fix dexing.

* gJF

Author: vkryachko

buildSrc/src/main/java/com/google/firebase/gradle/plugins/FirebaseLibraryPlugin.java

@@ -27,6 +27,7 @@
 import com.google.firebase.gradle.plugins.ci.device.FirebaseTestServer;
 import java.io.File;
 import java.nio.file.Paths;
+import org.gradle.api.JavaVersion;
 import org.gradle.api.Plugin;
 import org.gradle.api.Project;
 import org.gradle.api.attributes.Attribute;
@@ -48,6 +49,12 @@ public void apply(Project project) {
 
     LibraryExtension android = project.getExtensions().getByType(LibraryExtension.class);
 
+    android.compileOptions(
+        options -> {
+          options.setSourceCompatibility(JavaVersion.VERSION_1_8);
+          options.setTargetCompatibility(JavaVersion.VERSION_1_8);
+        });
+
     // In the case of and android library signing config only affects instrumentation test APK.
     // We need it signed with default debug credentials in order for FTL to accept the APK.
     android.buildTypes(