aaudio: use strong pointer to protect callback

Hold a strong pointer to the AudioStream so it cannot get deleted while
the MMAP callback is running.

Also fixes disconnect for shared streams, which was broken
by a previous attempt to fix b/166877425

Bug: 166877425
Bug: 170551030
Test: see bug for details
Test: atest AAudioTestCases
Test: OboeTester V1.5.30 or later, "TEST DISCONNECT"
Change-Id: I236e2d68632070d253d14ddf7b9d7076e068830a
Merged-In: I236e2d68632070d253d14ddf7b9d7076e068830a
(cherry picked from commit e358ec63)