[Media ML] Remove hidden API usages from MediaSessionService

It removes use of System.Secure.get/putStringForUsers
and the key.
This CL replaces them as getString/putString and
uses a context resolver for each user

Bug: 172793240
Test: manually confirmed dumpsys to check if MBR is restored
correctly for each user, cts test

Change-Id: I3a3f48506fefe87b9db18f32d8e2e68778e24100