Fix up accessibility issues in Time picker.

- btn_keyboard_key_material and btn_clock_material to have
white background without any alpha so that tinting works
propertly.
- Add 4dp more space in the time picker header so that PM
button has 48dp touch area.

Bug: 128664779
Test: visual inspection
Change-Id: Id7d8f51f37e95df1b63f1d8420b00ab724cf1a59