Implement `Toast#getXOffset()` and `Toast#getYOffset()` in the shadow

Calling the real offset methods results in a NullPointerException, as
`mTN` is null.  This implements the methods similarly to how `getGravity()`
is done.
Merge pull request #1413 from MeetMe/toast-getoffset

Implement Toast.getXOffset() and Toast.getYOffset() in the shadow.