Create style for AlertDialog in SystemUI

This creates the style up to the design spec and customizes the
following:
* Dialog icon on top of title
* Font sizes and colors
* Buttons
* Padding

Using R.style.Theme_SystemUI_Dialog for an AlertDialog (or preferably a
SystemUIDialog) will make sure that the correct style is applied.

Test: manual, using a test dialog
Fixes: 203666386
Change-Id: Ied39acb05295dea1e1e353ef504d013900a97266
Merged-In: Ied39acb05295dea1e1e353ef504d013900a97266