[OutputSwitcher] Grant BT permission to fix flaky tests

ag/33752573 introduce calling API which requires BLUETOOTH_PRIVILEGED, this causes MediaRouterServiceTests sometimes fail due to "Neither user 1010299 nor
current process has android.permission.BLUETOOTH_PRIVILEGED."

Test: atest
Bug: 425777921
Flag: com.android.media.flags.enable_output_switcher_personal_audio_sharing

Change-Id: I283ccd7cbbc672e2188293b7a625d677cd1c2a21