Clear calling identity before calling into AMS

Clear calling identity before calling into AMS in
unhibernatePackageForUser() to make sure we are calling as system user
and avoid security exception

Bug: 186701008
Test: built and flashed to wembley device, then run adb shell cmd
app_hibernation set-state PACKAGE_NAME false. No exceptions found.
Test: atest AppHibernationServiceTest

Change-Id: Icf0c72b85f4c611c84129a6fb9425f4344b4681b