Add DeviceDefault.System.Dialog[.Alert] themes

Customizable system default dialog themes.
Dark theme is preferrable on watch and TV.

Test: Flashed and tested on phone emulator.
      Same change applied to pi-tv-dev branch tested on an ADT-2.
      Used HeavyWeightSwitcherActivity as test dialog.
Bug: 133398453
Change-Id: Icc20555edcf78b78d9e57445faf4a99aaee9cbcd