Add keep rules for SysUI plugin annotations

These rules are necessary to ensure correct runtime plugin annotation
preservation for downstream targets built with R8 full mode. This was
previously addressed in downstream target keep rules, but with
aosp/2363992, such targets will now inherit the keep rules bundled with
the plugin library.

Bug: 215530220
Test: m + inspect downstream proguard_configuration output
Change-Id: I3793947827902617eaa5af681e99c889869cdca6