Introduce "Best Effort" shared UID migration strategy

Remove/disable all appId migration code in the system.

Introduce 2 levels of shared UID migration:
NEW_INSTALL_ONLY   = 1
BEST_EFFORT        = 2

The strategy to be used at the moment can be toggled by the system
property: "persist.debug.pm.shared_uid_migration_strategy".

The default strategy for now is BEST_EFFORT.

Test: atest CtsSharedUserMigrationTestCases
Bug: 220015249
Change-Id: If0e1148fdfd27a1dc4bf0066b2a661c6389488fe