Guard accessing `InfoMediaManager#mMediaDevices` with a lock.

Prevents reading incorrect values during the device list refresh.

Additionally, fix the unit test.

Fix: 417307066
Flag: com.android.media.flags.avoid_binder_calls_during_render
Test: atest com.android.settingslib.media.InfoMediaManagerTest,
Verified on a device.

Change-Id: Ic223ee3ccb9330d8115e23ab4b71c0569aa13a1d