Improve selection handle behavior for bidi text.

A point on a direction boundary can be mapped to two offset and
one offset on a direction boundary can be mapped to two points.
Previously, paragraph's primary direction is always used for deciding
offset and coordinates; thus, handle movement around a direction
boundary is often nonintuitive.

With this CL:
1. For selection end handle, direction of character at offset - 1 is
used for deciding handle shape and position.
2. For getting offset from coordinates, previous offset is used to
minimize the offset delta and primary .
3. For getting coordinates form offset, new logic chooses primary or
secondary horizontal coordinate depending on the current run
direction and paragraph direction.
4. When a handle passes another one because it passes a direction
boundary, new logic keeps the handle at the run boundary instead of
minimizing the selection.

Bug: 19199637
Bug: 21480356
Bug: 21649994


Change-Id: I2a7e87ad08416f4bd01a5f68e006240f77d9036b