Set PAUSING state when start pausing a stream.

When the stream is pausing, the data callback is already stopped. In
that case, there is not more data written by the client. The aaudio
service need to set the state as PAUSING so that it allows underflow in
the mixer. That can avoid misreporting underrun.

Bug: 336233924
Test: atest AAudioTests --instant
Change-Id: I128aa73a5c9a77eb6c5f11d0bf71b8325b1d4d0d