Fix bug #5037425 Canvas.drawText can't handle Right-to-Left text and text composing

- optimization for single run case was broken
- pass isRTL boolean along the call stack instead of the dirFlags integer
  (which was only used as a "isRTL" in the shaper)
- update unit tests

Change-Id: I33110b76a433633a0b92fbd1db03785204e0c3e6