Enable R8 for release builds

[mxalbert1996]

Overview (Required)

Enabling R8 optimization is strongly recommended for Jetpack Compose and there are significant performance differences between un-optimized builds and optimized builds (source). Enabling R8 shrinked the APK size from 14.0M to 4.4M and reduced first launch time by ~20% in my local test. There doesn't seem to be any problems with serialization after enabling R8.

[takahirom]

I thought I had it enabled, but hadn't! Thanks for finding this🙏