Fix failing MediaRouter2ManagerTest

The test failed for several reasons:
 - When other real cast device is present, tests are run before actual
   routes arrive.
 - Screen is not turned on during test.
 - Test lacks MEDIA_CONTENT_CONTROL permission.

This CL fixes the test.

Bug: 186190171
Test: atest com.android.mediaroutertest.MediaRouter2ManagerTest \
        --rerun-until-failure
Change-Id: I6f86ae7cba25871ffb7c8ee96f4c8db6bd86b816