Refine keep rules for Java classes w/ native methods

The global keep rule for native methods allows stripping of such methods
if they're unreferenced in Java. However, because system_server
explicitly registers these methods from native code, stripping them in
Java can cause runtime issues. As such, conservatively keep all such
methods in system_server subpackages as long as the containing class is
also kept or referenced.

In practice, the difference is minimal, but this makes it easier to
integrate optional debug pathways that have JNI interop.

Bug: 246800015
Test: m services + inspect delta in services.jar
Change-Id: Id1f9ab707edff9b1911cdde6bdd4bc24ebff7e49