Only keep Parcelables if used

Use `-keepclassmembers` instead of `-keep` for Parcelable types
and their CREATOR members. This allows shrinking of Parcelable types
that aren't referenced while still preserving the necessary CREATOR
member when the classes are kept. This saves ~3MB across various
platform targets.

Bug: 238670321
Test: m + presubmit
Change-Id: I13508073dbc07e28aa93bb6604c8f838bcf72646