Apply contextStart consistently in getRunAdvance

The contextStart needs to be applied to all offsets relative to the
actual start of context. The code was missing "offset", which caused
mischief especially in mixed LTR and RTL text.

Bug: 21573666
Change-Id: I47a1b6cde5862442b9c7236ee72b2ceb0df9b2e9
1 file changed