Restrict optional system_server optimizations to shrinking

For the initial rollout of system_server R8 optimizations, we will
only apply code shrinking. This avoids the need for retracing in core
developer workflows, while providing modest benefits through code
shrinking.

Shrinking is still enabled in an opt-in basis for specific devices
via `SYSTEM_OPTIMIZE_JAVA=true`.

Bug: 210510433
Test: m SYSTEM_OPTIMIZE_JAVA=true
Change-Id: Ie9fb8138afd0cc60799e3154c709630a947968c0