audio: Apply app type gain at start of stream

App type for a usecase is derived at start of stream.
Therefore, any volume gain based on app type cannot be
applied before start. Cache any gain value
sent from AF before start of stream and send after
stream has started

Bug: 62393776
Test: hangout call
Change-Id: Ia772d5ee1b2cc7293d69b31f06f1c160a5d7284e
Signed-off-by: David Lin <dtwlin@google.com>