Fix padding and touch target size on BannerMessage.

Padding between title and subtitle is 8dp should be 4dp.

Touch target on dismiss button is too small for a11y requirements. Use
a touch delegate on the card view to increase touchable size without
changing layout.

Bug: 190780536
Test: Manually confirmed new layout works
Test: Use adb shell input tap x y to touch dismiss button
Change-Id: I749ab2e7cba811d841804d73c13a2b792591dfa1