Allow for X and Y offsets in GUI

These can be used to center a lower resolution theme onto a
higher resolution screen. May be useful for Nexus 9 unless or
until we decide to do a 1536x2048 theme or on devices like the
Moto 360 and LG G Watch R.

The build flags will only affect the stock theme. If a custom
theme wishes to apply an offset, it can be done by setting
tw_x_offset and / or tw_y_offset in the variables section of the
XML.

Change-Id: I2390769ed861b96a4a3bb8a0f06d9021ec91b6b9