MediaRoute: update mock to contain addr

This CL updates the mock to create the audio device with address, and
ensure that the address is empty string rather null due to that being
the default and to comply with existing non-null annotations.

Bug: 419589719
Test: run presubmit with `if (flag)` -> `if (true || flag)`
Flag: com.android.media.flags.enable_device_address_as_identifier_in_media_router_2
Change-Id: I1b71a7e0104dddc8e82cc53f0dd62dc5d7376fc6