AudioFlinger: reset frame position on standby

The HAL does not reset the frame position on standby().
But applications expect the frame position to be reset.
So we subtract the position at standby from the current position.

Bug: 21724210
Bug: 21930805
Change-Id: I0c4520ba1c6c06a580f45f6bafc8cf1d56969f07
Signed-off-by: Phil Burk <philburk@google.com>