SystemUI: add top exclusion to the back gesture

Allow the user to exclude the top ?dp of the device from the
back edge gesture area

Change-Id: I1f3c07706d4121461395af568ac4662b942d8488
Signed-off-by: Joey <joey@lineageos.org>
Signed-off-by: Luca Stefani <luca.stefani.ge1@gmail.com>
Signed-off-by: Jesse Chan <jc@lineageos.org>