Add more firstStrong alternatives for textDirection.

The current firstStrong value of textDirection is locale-dependent,
and can create problems when consistent display of text across
locales of potentially different direction is desired.

This adds two new values for textDirection, firstStrongLtr and
firstStrongRtl, which don't use the locale or the view's direction as
fallback, but explicit values of LTR or RTL.

Using firstStrongLtr also guarantees the exact algorithm defined in
the Unicode Bidirectional Algorithm to be used for determining
direction.

Bug: 13428339
Change-Id: I450d4f6b5197763ace7f9506c72cf87a90da4218