Clean up dirFlags / bidiFlags confusion

The dirFlags and bidiFlags enums are distinct, and have different
meanings. The former is a determined direction for a run of text, while
the latter is a request for the bidi algorithm. They have been used
interchangeably, and this has caused some problems, notably running the
bidi algorithm needlessly when the direction for a run is already
determined.

This patch cleans up the confusion, by always naming each occurrence
explicitly "boolean isRtl" or "int bidiFlags" (the previous code often
just used "int flags", which added to the confusion), and converts
between the meanings when a function takes an isRtl argument but passes
it to another function expecting bidiFlags.

Fixes b/15089607 Clean up bidi flag mess

Change-Id: I410b6604376e853dd12c255e7f5a9d2b9a310dd9