Fix proguard for release build.
Fix proguard for release build.

Proguard return value was not accurate.  Changed to a wild card
so that the difference between Unit and void will not cause issues.

Bug: 213932665
Test: Run sample app with minify enabled.
Change-Id: Ic705f42c87e36fbca2409baa4daa20fc70244b66

AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx