System MR2: Check MODIFY_AUDIO_ROUTING permission

This CL adds a permission check for
MediaRouter2#getInstance(Context, String)
which creates the system MediaRouter2 instance.

Bug: 183428114
Test: CTS only passes with the right permission.
Change-Id: I38561dd6525d9e3c4514829662483d2c7a08dda1
5 files changed