feat(android): use proguard files

[m1ga]

Currently you can't use proguard.cfg files during the build process. I've enabled this.proguard = true; for the release build. If there is a proguard.cfg in the app/platform/android/ folder it will set it to proguardConfigFile which is used in the gradle files later.

If there is no file or if you do a test/debug build it behaves the same as before.

I remove the error/exit part. We should add a section in the docs where to put the file if needed.