Began implementation of Dialer dark theme.

 - README on how to properly theme Dialer going forward.
 - Migrated all widgets to use global colors.
 - Removed all activity and application themes where it wasn't necessary.
 - Added themeing test rule for Espresso tests.

Bug: 79883035
Test: tap
PiperOrigin-RevId: 197634256
Change-Id: I4b7d94d45aeeb59d484b0069fdd1e200a654910b