Increase app header height to 48dp

From 40dp, to comply with a11y minimum touch target requirements.

Flag: com.android.window.flags.enable_tall_app_headers
Fix: 397592755
Test: verify app headers now have a 48dp height and buttons, dragging
and customizable headers are WAI.

Change-Id: I5f8ad5619905d39b15d8b2ce7de794784a1d13c9