Fix output switcher didn't show album art

Before this CL, output switcher will show application
icon when the meta data is null.

In this CL, output switcher will align the behavior of
the media object. If the meta data doesn’t have an album
art, then create it based on notification icon.

Bug: 162998388
Test: atest MediaOutputControllerTest
Change-Id: I761d83af8e08a744e4cd5c55ff2c445125885730