Increase size of the back/ime switcher buttons in gesture nav

- This matches the size of the back/ime switcher buttons in
  button nav. Previously we intercepted all touches in the nav
  bar, but since the IME is interested in handling touches in
  the bar (where there are no buttons), we moved to specific
  touch regions for each button. As a result, only the immediate
  button size was tappable making the target too small.
- Also make the parents in the hierarchy non-clipping so that
  it doesn't clip the background ripple with the larger size

Bug: 184983443
Test: Show the IME, verify the buttons are larger
Change-Id: I4db6df26d8a1e1bd6675635224baa9a9ee6de044