Use ImageButtons instead of Buttons in handle menu

Using <Button> + android:background to set the icon drawable
results in the android:padding attribute having no effect. That
meant that the touch target for handle menu windowing buttons
was effectively 32dp (its width/height).
By using an ImageButton + android:src, padding actually does take
effect, so the touch target is now 40dp (its width/height) and
the icon remains the same size (32dp) due to the 4dp padding.

Bug: 270412904
Test: manual - caption windowing buttons are same size, but click
target is slightly bigger

Change-Id: Ia1e77533fb65630e9eaf6108dce7cf5240c176f7