Use top fullscreen window to inherit SystemUI flags

The top fullscreen window of an app token is the one we use for
determining SystemUI flags normally. Thus, when showing the
snapshot, we need to inherit the flags from the same window.

Test: Open event in Calendar, go home, reopen Calendar app,
observe no status bar icon color change.
Test: go/wm-smoke

Change-Id: I96678000339617c8fc51e72c0f6e2e167f542491
Fixes: 62871307