AudioPolicy: manager: fix patch list growing up

commit 3b17354a caused potential stale audio patches
records in audio policy manager for patches associated
with audio flinger software patches sharing an output mixer.

Bug: 136121584
Test: dumpsys media.audio_policy &&
adb shell /data/nativetest64/AudioPolicyEmulatorTests/AudioPolicyEmulatorTests --gtest_filter=AudioPatchTest/AudioModeBridgingTest.
&& dumpsys media.audio_policy:
ensure list of patches did not grow up
Test: atest audiopolicy_tests

Signed-off-by: Francois Gaffie <francois.gaffie@renault.com>
Change-Id: Ife8035dc2d2d9c0b568d6847ca5eb0d2cba77cea