Fix various conformance issues with TextDirectionHeuristics.

1. The algorithms no longer consider directional embeddings and
overrides as strong characters.

2. The algorithms are updated to Unicode 7.0, and consider the
directional isolates in determining the direction of input.

3. Codepoints are used instead of code units for looking up bidi
properties.

4. The bidi direction of unassigned characters is considered.

Bug: 20142480
Change-Id: I9be161c112b8f23565ed8961bb7d44ced234f67a