Run screen capture requests asynchronously

After posting the screenshot request to the main thread, don't wait for
the results and instead just return immediately. The results from the
screenshot will be sent through the requested ScreenCaptureListener

In the current change, SurfaceComposerClient still waits for the
result before returning so the JNI callers still get the screenshot
synchronously. The call will become asynchronous in native in a later
change.

Test: SurfaceFlinger_test
Test: adb shell screencap
Test: power + volume down screenshot
Test: Recents screenshots
Change-Id: I5e352d6920b9298a0a376354f977f2ef7456841b