audio policy: fix crash if output for DAP is not opened

Fix crash in getOutputForAttrInt() if the output corresponding to
the mix returned by AudioPolicyMix is not opened and the device
selected by the mix is not attached.
Before commit c529cf6f, a policy mix without opened output would not
be retuned by AudioPolicyMix.getOutputForAttr(). With direct
output suppoort, this is possible but should be ignored if the device
selected by the policy mix is not attached.

Bug: 155430290
Test: AudioHostTest
Test: AudioServiceHostTest
Test: audiopolicy_tests
Test: manual mirroring and car projection tests
Change-Id: I66be4a3c5fb8b9ae0674c0f1e6e39de7f18ed2fb