SurfaceFlinger: move sync_wait for screen capture to client

Free up time from the SF's main thread by moving the fence waiting
to the client.

Test: Observe systrace of region sample thread
Test: adb shell screencap
Test: Recents takes the screenshot
Test: Rotate device
Test: Volume + power down for screenshot
Bug: 178649983
Change-Id: I0a4991c013375b1f354e0728a06ca30a835b0422