audio: Use flag instead of negative size hack to indicate exclusive mode support

Bug: 131325988
Bug: 134634278
Test: use Oboetester_20190424.apk

Change-Id: Ic7dec7a38ee11e541cab6eccf1dd49a0d40d2533
Signed-off-by: juyuchen <juyuchen@google.com>
(cherry picked from commit fc2ff82d9f20deee96448e5ba500dca0601f3e13)
Signed-off-by: Aniket Kumar Lata <alata@codeaurora.org>