Move call to MediaRouter2Manager to worker thread

The call to MediaRouter2Manager#getRoutingSessionForMediaController was
implicated in an ANR. So, moving the call to a background thread to
prevent it from being caught blocking the main thread.

Fixes: 161690149
Test: atest tests/src/com/android/systemui/media/MediaDeviceManagerTest.kt
Change-Id: Icf3945a971be84397a335672134b1067decfe6da
(cherry picked from commit 290a6244)
Merged-In: Icf3945a971be84397a335672134b1067decfe6da