improve text selection

improve text selection

LayerAndroid.*
Adjust the hit-test (x,y) by the layer position.
Keep track of the adjusted (x,y) and store the best
to be returned when the best layer is found.

CachedRoot.cpp
Remove code that adjusted (x,y) by layer bounds.

SelectText.*
Detect columns of text and prefer new characters in the same
column as the existing selection.

Don't extend the selection until the tap point moves past
the word anchors.

There's more work to do on selecting text in layers.

bug:3275625
bug:3271730
bug:3191699
Change-Id: Ib3c2b35e5eebe30c6032f484cf76d388e94293e0