Move NavButtons to end of taskbar

* Layout hotseat icons from the end instead of the start
* IME down and system back arrow are now separate views

Bug: 191399224
Test: Tested w/ RTL + LTR in gesture
and 3 button nav w/ and w/o IME

Change-Id: I4d0ecd0bee0c519892c63eeefef45055b26d349b