Make sure touch only goes to the nearest button when it makes sense

- In NearestTouchFrame, we can't use view bounds alone since the buttons do not have the same root (3 buttons are in the main container, while the contextual buttons are in the contextual button container). This currently caused the issue that contextual button region, when the contextual buttons are invisible, triggers overview
- Need to add an empty space in the left / top area of the button nav to prevent view clicks from going into back (that's how it works in navigationbar/)

Bug: 25768138
Test: left and right of 3 buttons don't trigger back or overview
Change-Id: Idc26c0c8ac0ecc000300a6db2e3e6251f678dada