Only use system gesture insets for exclusion rects with root ime view

- Instead of using the whole IME frame, we should only exclude the
  left and right regions for excluding the back gesture (the intention
  of the original change), otherwise the full IME frame will exclude
  the bottom gesture area which is respected by Launcher to prevent
  quickswitch.

Bug: 171501996
Test: Dump exclusion rects received by SysUI with normal IME (only edges)
      floating IME (no exclusion requested) and extract mode (only edges)


Change-Id: Id8e01d56190f8fafdc2da1cf95203e597acdb970