Enable R8 full mode by default for apps

This provides significant size savings over the default compatibility
mode, roughly ~20MB in aggregate for a recent Pixel build. Targets can opt out using `proguard_compatibility = true`.

Future work will enable for all targets, not just apps, but this
incremental change yields the most benefit.

Bug: 215530220
Test: m + presubmit
Change-Id: I52a9b27ebe71835849e200e23ae2a250eaf1bbe1