SF: Add WindowDisconnector for screenshot code

Adds a simple WindowDisconnector class that holds an ANativeWindow* and
disconnects from it when it goes out of scope. This allows us to
drastically reduce the nesting of the captureScreenImplLocked method
of SurfaceFlinger.

Bug: 62257775
Test: SurfaceFlinger_test and manually verified that screenshots still
      work
Change-Id: I55123a7a6b2036158d0959328b0e6f8b206cce5d