Set gesture exclusion rect for IMEs

Use visible inset values provided by
IMEs to set gesture exclusion rects
for the EdgeBackGestureHandler to ignore
regions where the keyboard is.
If the IME has not overridden
onComputeInsets(), InputMethodService
uses the location of R.id.inputArea
to approximate the location of where
the IME region starts.

Fixes: 141215181
Test: Tested full screen landscape keyboards
(Messenger, Hangouts), non full screen landscape
keyboards (SMS Messages), searching from the top
of the screen in the Toolbar (Google Play Store)

Change-Id: I359d719493fb92d49cd309c2d00371134cd758fe