Move the window background to the activity theme

This has several advantages:

- Reduces overdraw and improves performance especially on weaker and
  older devices

- Centralizes the background attribute out of the layout files

- Shows the proper solid background when the calculator is launching

Change-Id: Id3a4f7499a8bfc50f6b0ff8307d9d420861e0858
Merge "Move the window background to the activity theme" into cm-10.1