Audio HAL VTS: refactor prepareFor{Reading,Writing}

Those functions had lots of copy paste on errors and
the following patch will even add more error detections.
Refactor the hidl_cb call to avoid all duplication.
Note that both function should be refactored as 99% identical.

Test: vts-tradefed run vts --module VtsHalAudioV2_0Target
Test: call/play music/record/video...
Bug: 36311550
Change-Id: I40d6926b4f9f5e3aba51e878f55fb013f4ca09c1
Signed-off-by: Kevin Rocard <krocard@google.com>