Prevent SecurityException during AVB unit tests

Mock out displayOsd because it broadcasts an intent, which causes a
runtime error due to missing permissions on ADT-4.

Test: atest TvToAudioSystemAvbTest PlaybackDeviceToTvAvbTest
PlaybackDeviceToAudioSystemAvbTest

Bug: 289353130
Change-Id: I5a45c62d12938d9c4339a51e849979ed71f4220c