Add Crashlytics suffix to keep filename to avoid clashes

[pswaminathan]

It's not uncommon to have a keep file in consuming apps. In my own case, this resulted in:

CONFLICT: raw/com.ergatta.bolt.keep is provided with ambiguous priority from:
	src/main/kotlin/com/ergatta/bolt/res/raw/com.ergatta.bolt.keep.xml
	bazel-out/darwin_arm64-opt-android-ST-4cf3fc44044d/bin/src/main/kotlin/com/ergatta/bolt/_crashlytics_crashlytics/res/raw/com.ergatta.bolt.keep.xml

This makes it unique.

[ahumesky]

Thank you!