AudioService: fix AudioProductStrategy matching rule

Strategies use rules on attributes, and use public getter
to compare usages. However, some usage are system only and
now filtered by public getter.
This CL fixes this issue by adding a hidden get for usage
that return whatever public or system usage.

Test: adb shell am instrument -w -e class com.android.audiopolicytest.AudioManagerTest#testAudioAttributesToStreamTypes com.android.audiopolicytest

Signed-off-by: Francois Gaffie <francois.gaffie@renault.com>
Change-Id: Iddd55af8f230cb063dcaa9ba950b890cbbb81643
Merged-In: I377e840b18443c33fb0b562b27fbc706aaeb8055