Do not revoke pre-M perms on upgrade

... just remove the REVOKE_ON_UPGRADE flag.

Pre-M apps do not know how to ask for permissions. Hence only the user
can revoke permissions as this is likely to break apps.

Test: atest CtsPermissionTestCases:SplitPermissionsTest
Change-Id: I4f7634985cb655f6b8f98ecf4261e1afcb4519de