Index out of range problem in TextLine.

Recent refactoring for bidi introduced an index shift in the getOffsetBeforeAfter
method. This problem appears for multi-line text input only, when the text line
mStart index is not 0.

As a result, moving the cursor using the trackball in a multi-line EditText crashes.

Change-Id: I1f121f0f9272ef7d338399f369ba6d77e1ca71c5