Fixed AudioMixingRule usage of AudioAttribute#getUsage

Given that system usages were added to the AudioAttributes, system
components need to use the API were applicable. AudioMixingRule was
missing this and thus the audio attribute usage base routing did not
work correctly. In particular the registration of the audio policy mixing
rules would not register the system usages based routing correctly.
Instead, the match usage rule would default to unknown usage instead of
the requested system usage.

Bug: 186661937
Bug: 186442942
Bug: 186158497
Test: build gcar_emu_x86 and run car kitchen sink to make sure routing
    performed as expected.
Change-Id: I4aaedf1b6b9144bd2e84d2d7112b5ebcf744a8b2