Camera: Exit PreviewStreamSpacer when disconnecting stream

- Call requestExit() during Camera3OutputStream disconnect.
- Remove requestExitAndWait() from the PreviewStreamSpacer
  destructor.
- Return false if exitPending() is true in threadLoop.
- Use wp<> when accessing parent from PreviewStreamSpacer.

Test: Camera CTS, and vendor testing
Bug: 232046746
Bug: 232829166
Change-Id: I136a93fee37cd6fbdb8050cacea1511170e03ee6