Update top of modes page

* Show blurb based on mode type.
* Make the icon bigger and highlight it when mode is active.
* Increase spacing between elements.

Also eliminate some code duplication between header of mode page and header of icon picker.

Fixes: 355415875
Test: manual
Flag: android.app.modes_ui
Change-Id: I7e788b9b5920cedb791d1571b19b37e65ece6d0b