audio policy: fix mmap behavior on device disconnection

Commit 8e70cf81 changed the behavior od an AAudio MMAP stream when
playing over built-in speaker and a wired headset is connected.
This change restores previous behavior while preserving the fix for
b/345044066.

Bug: 345044066
Bug: 338143144
Bug: 349622795
Flag: EXEMPT bug fix
Test: Oboe Tester TEST DISCONNECT
Change-Id: I905585b718c298b0dabefacff4f667f7e4981572