Support theme width and height offsets

Current use condition is LG V20, where the secondary screen is
actually just a corner chunk of the main screen.
In this case, we need to shift the UI down some pixels in order
to see it all, but just using Y offsets resulted in losing
the rest of the GUI below.

Example in BoardConfig.mk:
  # Shift TWRP off the secondary screen
  TW_Y_OFFSET := 160
  TW_H_OFFSET := -160

Change-Id: I5a06638ba0d05e5b9fd8a6245c630d6ea3394f78