Support leaving sharedUserId in PackageManagerService

This is part 2 of ag/15290670

- Return the proper error code when sharedUserId mismatches
- Update Settings.updatePackageSetting(...) to also handle the case when
  a package is leaving a shared UID group
- In Settings.updatePackageSetting(...), update PackageSettings with new
  SharedUserSetting so changes will be picked up during the commit stage
- Introduce new CTS test: CtsSharedUserMigrationTestCases

Test: atest CtsSharedUserMigrationTestCases
Bug: 179284822
Change-Id: Iaf984eed7fd45238b42020789b3b14e2dc4415be