Add selection handle dragging tests.

- Set an id for each HandleView to access the hanlde.
- Rename DragOnTextViewActions to DragAction to use it for
dragging handles.
- Introduce HandleCoordinates to provide proper coordinates
for handles.

More tests will be added in following CLs.

Bug: 25730231
Change-Id: I9276bf2f983983ec9aae0ddcf674d3dcee566892