Increase the expand button touchable area.

The touchable area used to be a 48x48 square, but we
actually want the entire right-hand side of the
notification to be touchable in the collapsed state.

Bug: 163626038
Test: atest SystemUITests
Test: manual - check touch bounds and validate w/ TalkBack
Change-Id: I9c1b5a0e01d841ed5d448ee5e1030b7f1cbbb2f2