Enable R8 full mode for system_server

This is the default for all app targets. Incrementally enable for
select library targets that are already being shrunk or optimized w/ R8
in anticipation of a broader default rollout for any optimized target.

Bug: 215530220
Test: m + presubmit
Change-Id: I17781fa5fa27f453694fbdd20e11f7ce387dede7