This patch extends the previous BiDi support (when the direction for the
entire string was given by the caller) to run the BiDi algorithm
(provided by ICU) over the string to break it into BiDi runs. Thus, it
handles mixed LTR and RTL strings in a single layout, and also respects
heuristics for inferring the paragraph direction from the string.

Change-Id: Ia4b869de3c139c5a7d16b8ce7766870b98a815ea