Add PackageInstaller#uninstallExistingPackage

This new API allows an app to be uninstalled silently by any app holding
the DELETE_PACKAGES permission, as long as the app is installed in
another user so won't be fully removed from the device.

Bug: 149601842
Test: atest UninstallExistingPackageTest
Change-Id: I69fe4d1dd4e9da83574b431257f7be6d1ac8b2bb