Give flag to R8 if we explicity opted for optimized shrinking

This allows us to keep R fields for targets that are not explicilty opting in (many tests rely on these, and R8 shake them away)

Bug: 372264901
Bug: 325905703
Test: v2/android-virtual-infra/test_mapping/presubmit-avd
Change-Id: Ifeede36f6b6fae5ebc382e38ca2eba6725c0e84b