AudioFlinger: FastCapture: fix onWork segfault

Bug: 305787645
Test: atest AudioRecordTest

This CL fixes a seg fault on FastCaptrue onWork when a capture
is closed directly. Stop is called, followed by close.
The fast capture is first paused, then may be closed before
the FastCapture thread has even received new state from record thread.

This CL keeps a ref on the fast capture until fast capture thread
has send back the ack.

Change-Id: I939d601d2e8afe176b959e3cd755f41b2e835439
Signed-off-by: François Gaffie <francois.gaffie@renault.com>