pm: PackageBackwardCompatibility -> applyPolicy

This moves PackageBackwardCompatibility from parsePackage to
applyPolicy.

The motivation of this is to expose partition information to these
files. Specifically, to expose whether the app is a pre-installed
(or updated pre-installed app) for certain backwards compatibility
rules that should only apply to these cases.

Bug: 86259915
Test: boot Pixel 2
Test: Pixel 3: atest -p cts/tests/signature
- all passed except for CtsSystemApiAnnotationTestCases test which
  failed (known issue b/124210145)
Change-Id: Ic824f06489781233dc98bf3dc0182681d32f7142