oboeservice: separate mDisconnected from mState

The DISCONNECTED state was preventing the pause and stop
from finishing. The DISCONNECTED status is really orthogonal
to the other states like STARTED and STOPPED and needs to
be tracked separately.

Bug: 185586171
Bug: 182852602
Test: atest CtsNativeMediaAAudioTestCases
Test: OboeTester "TEST DISCONNECTS"
Change-Id: Ie0e767819524271694170918b2d525d5b1da3940