Clean up PropertyInvalidatedCache for SystemApi

Bug: 152453213
Tag: #refactor

This commit prepares PropertyInvalidatedCache to function as a system
api.  The changes may be summarized as follows:

 1. Visibility changes - some formerly protected or public methods are
    now private.

 2. Use of ParcelFileDescriptor instead of FileDescriptor.

 3. Null-ness annotations have been added to public methods.

 4. An onTrimMemory() method has been created that hides some class
    internals from ActivityThread.

An existing test (os/PropertyInvalidatedCacheTest) has been deleted.
Its contents have been merged into app/PropertyInvalidatedCacheTests.
The deleted test may never have been used.

Test: atest PropertyInvalidatedCacheTests

Change-Id: I619e715c3deca016b21c69bf34a8e61771957e68