android: create build flavor for omitting non-free Google dependency

The F-Droid app store don't support non-free dependencies. Create two build
flavors, "fdroid" for building without Google Sign-In, and "play" for including
it.

Modify Makefile to target the play flavor.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

Makefile

@@ -19,15 +19,15 @@ all: $(APK)
 $(DEBUG_APK):
 	mkdir -p android/libs
 	go run gioui.org/cmd/gogio -buildmode archive -target android -appid $(APPID) -o $(AAR) github.com/tailscale/tailscale-android/cmd/tailscale
-	(cd android && ./gradlew assembleDebug)
-	mv android/build/outputs/apk/debug/android-debug.apk $@
+	(cd android && ./gradlew assemblePlayDebug)
+	mv android/build/outputs/apk/play/debug/android-play-debug.apk $@
 	
 $(RELEASE_AAB):
 	mkdir -p android/libs
 	go run gioui.org/cmd/gogio -ldflags "-X tailscale.com/version.Long=$(VERSION_LONG) -X tailscale.com/version.Short=$(VERSION_SHORT) -X tailscale.com/version.GitCommit=$(TAILSCALE_COMMIT)" -tags xversion -buildmode archive -target android -appid $(APPID) -o $(AAR) github.com/tailscale/tailscale-android/cmd/tailscale
 
-	(cd android && VERSION=$(VERSION_LONG) ./gradlew bundleRelease)
-	mv ./android/build/outputs/bundle/release/android-release.aab $@
+	(cd android && VERSION=$(VERSION_LONG) ./gradlew bundlePlayRelease)
+	mv ./android/build/outputs/bundle/playRelease/android-play-release.aab $@
 
 release: $(RELEASE_AAB)
 	jarsigner -sigalg SHA256withRSA -digestalg SHA-256 -keystore $(KEYSTORE) $(RELEASE_AAB) $(KEYSTORE_ALIAS)

android/build.gradle

@@ -32,12 +32,24 @@ android {
 		sourceCompatibility 1.8
 		targetCompatibility 1.8
 	}
+	flavorDimensions "version"
+	productFlavors {
+		fdroid {
+			// The fdroid flavor contains only free dependencies and is suitable
+			// for the F-Droid app store.
+		}
+		play {
+			// The play flavor contains all features and is for the Play Store.
+		}
+	}
 }
 
 dependencies {
-	implementation 'com.google.android.gms:play-services-auth:18.0.0'
 	implementation "androidx.core:core:1.2.0"
 	implementation "androidx.browser:browser:1.2.0"
 	implementation "androidx.security:security-crypto:1.1.0-alpha02"
 	implementation ':ipn@aar'
+
+	// Non-free dependencies.
+	playImplementation 'com.google.android.gms:play-services-auth:18.0.0'
 }