Audio hal VTS: factorize master volume test

Master volume was the first pair of getter/setter tested
and did not use the generic test*Accessor function.

Test: run the corresponding vts test
Test: vts-tradefed r vts-hal-hidl -m VtsHalAudioV2_0Target

Bug: 34170075
Change-Id: I77080a3a17e4a3e97ecb72a8a89ad2d88626d96f
Signed-off-by: Kevin Rocard <krocard@google.com>