Expose drawTextRun publicly

For correct low-level drawing of low level text, a method that
includes context for shaping is necessary, and it's similarly useful
to provide the direction explicitly rather than running the BiDi
algorithm on the text. The drawTextRun method (in both char[] and
CharSequence variants) has provided this functionality for several
major releases but has been internal. This patch exposes the
methods publicly, and also improves the doc strings for both
the new method and some related ones.

Bug: 20193553
Change-Id: I9be33ca5ae3e7db2b69a56298400671d5ef8ad05