Move PlatformCompatCache to new package

We plan to separate the OomAdjuster related code into a separate package
for better encapsulation and clear interface.

This CL creates the new package `com.android.server.am.psc` and moves
the first class PlatformCompatCache.java into it.

Bug: 425766486
Test: mmm frameworks/base/services/
Test: atest MockingOomAdjusterTests
Flag: EXEMPT pure refactor

Change-Id: I8941080a48b32faa605fed39193b0a5005665595