force use of fdroid flavor for development

apparently, the google version hits the method limit. there is no reason
to build the google version for debugging, so might as well just limit
that (and document it) to avoid confusing developers who are new to the
code base.

see https://github.com/open-keychain/open-keychain/issues/2124

OpenKeychain/build.gradle

@@ -252,6 +252,12 @@ android {
         }
     }
 
+    variantFilter { variant ->
+        if(variant.buildType.name.equals('debug') && variant.getFlavors().get(0).name.equals('google')) {
+            variant.setIgnore(true);
+        }
+    }
+
     /*
      * To sign release build, create file gradle.properties in ~/.gradle/ with this content:
      *

README.md

@@ -54,6 +54,8 @@ Select SDK Platform for API levels 25.
 6. Execute ``./gradlew assembleFdroidDebug``
 7. You can install the app with ``adb install -r OpenKeychain/build/outputs/apk/OpenKeychain-fdroid-debug.apk``
 
+The "google" flavor is only used to include donations via Play Store, for development the "fdroid" flavor should be used.
+
 ### Run Tests
 1. Use OpenJDK instead of Oracle JDK
 2. Execute ``./gradlew clean testFdroidDebugUnitTest --continue``