Fix flicker with "trampoline" activities

WC.mSurfaceControl starts hidden, but gets immediately shown
in onParentSet. Reflect that in mLastSurfaceShowing in
AppWindowToken such that it gets hidden immediately again while
preparing the first frame.

Test: Go to settings -> Add account, observe no flicker in the
status bar
Test: go/wm-smoke

Change-Id: I6493e75fcac31784662ebcb0e24a4e5c698ed337