Refine system-server Proguard config for shrinking

This CL yields a basic, if unoptimized, set of Proguard rules for
shrinking system_server. These fall largely into three categories:
 * Code referenced via JNI
 * Code referenced via explicit reflection (runtime .jar loading)
 * Code referenced via implicit reflection (binder/services/etc...)

Follow-up work will tackle specific areas to make the rules more
maintainable and compact as we work to enable shrinking by default
for system_server.

Bug: 210510433
Test: SYSTEM_OPTIMIZE_JAVA=true m + manual testing
Change-Id: Iea3182c0536dbf7597e452f96cc33b4ab71dce96