audioflinger: fix mmap device selection

Preserve initial device selection on mmap streams in order to
reuse it when a new client starts on the same stream so that
audiopolicy does not use the default route for the new client.

Bug: 64981153
Test: verify AAudio playback over MMAP with forced device selection to
speaker while headset is connected.

Change-Id: I7c117c3c0f8e7ad08f8e63ea804c19ce21808874