aaudio test: add -s to test_steal_exclusive

Add -s option to request SHARED mode.
This will not cause a steal. But it can be used
to test the response to a disconnect.

Bug: 158316262
Test: test_steal_exclusive -r0 -d0 -s
Change-Id: I209dc72028ce61881a6d2d14a9cec689a2d74264