Add proguard rule to keep com.google.android.datatransport.Priority.

The enum's values are accessed via reflection, so aggressive Proguard
rules can cause it to be stripped out.
e.g., #1996

Author: mrichards

transport/transport-api/proguard.txt

@@ -1,2 +1,5 @@
 -dontwarn com.google.auto.value.AutoValue
 -dontwarn com.google.auto.value.AutoValue$Builder
+-keepclassmembers enum com.google.android.datatransport.Priority {
+     *;
+ }