Make the caption window spy when requesting custom headers

The INPUT_FEATURE_SPY had been removed and replaced by custom caption
touchable regions because the latter was an a11y-friendly alternative.

It turns out that while that is still true (a11y services can see the
custom content when setting touchable region carveouts but not when
just making the caption window spy), removing the spy feature also
caused a regression when the app's top window is transparent or
semi-transparent modal. In such cases, the caption is still transparent
and the custom content is still visible through the caption and through
the modal, so the intended UX is that touching any area of the
customizable caption would dismiss the modal, but the custom touchable
region in this case is not set because the modal prevents the gesture
exclusion regions from being reported to SystemUI.

By making the window spy, we let the first motion event fall through
before pilfering, which is enough to dismiss the modal through the
caption window.

Also fixes a bug in wd2 where |checkTouchEventInCustomizableRegion| only
worked for AppHandleController but not AppHeaderController.

Flag: com.android.window.flags.enable_accessible_custom_headers
Fix: 414521306
Test: Open chrome in desktop with custom headers, open the 3-dot menu on
the top-right of the search/toolbar, then tap on any part of the
customizable caption region, customized or not (so a tab or the empty
space) and verify the 3-dot dialog menu is dismissed.

Change-Id: Ia5ac6b2658145dfb18cc27ad3fce1647e0714dbc