Add adoptShellPermissionIdentity to setup

The test passes locally, but fails remotely. The failure is
due to lack of MODIFY_AUDIO_ROUTING, so for some reason the
permission is granted by default locally but not remotely.

This change makes the test pass both locally and remotely.

Bug: b/326226444
Test: atest MediaRouterServiceTests
Test: abtd run L65500030002106822
Change-Id: I2afca7a0e15cdcba89bab3075731a05adfa37c29