Set the global priority session immediately after its flag is set

The MediaSessionService has set the global priority session only after
the session becomes active for the first time.

From the bug reports, com.android.settings (shares the UID with the
global priority session) made sound before that. So the global priority
session became the media button session before the service handle it
specially.

This CL fixes the issue by moving the global priority session
immediately after the FLAG_EXCLUSIVE_GLOBAL_PRIORITY is set.

Bug: 67322829
Test: Run CtsMediaTest and CtsMediaHostTest.
      Also did the manual test described in the issue.
Change-Id: Iea9543b00a783b32dbc26f682d2352068c69e413