Support U+061C ARABIC LETTER MARK by making it invisible when rendering text.

The character works automagically otherwise: for the Unicode Bidirectional
Algorithm, the default type for undefined characters in the Arabic block is
set to AL.

Patch cherrypicked from upstream at:
https://github.com/behdad/harfbuzz/commit/a8bf0e91f18341e1e20f4e3341fc7dcdd0c990e1

Bug: 10190564
Change-Id: Id50c3163d3751b0247095c1c110432962683638c
Merge tag 'tags/android-4.4_r1' into kitkat-staging

Android 4.4 Release 1.0