Clickable spans - trigger onClick onSingleTapUp

Previously (post Android N) we decided to introduce
a gesture detector in textView and trigger the onClick()
over a clickable span when onSingleTapConfirmed was
called. The problem is that onSingleTapConfirmed also
waits for double tap timeout to make sure (to confirm).
This has a chance to break some apps so relax the
rule to only look for onSingleTapUp.

* the bug for which the initial change was made is
still fixed
* slightly amended the ClickableSpan JavaDoc to
specify that the onClick gets triggered if the
span is selected when clicked (note that this
behavior is unchanged since ClickableSpan exists,
added just an extra hint to devs)

Bug: 23692690
Test: in the attached CL (same topic)
Change-Id: Iead434aa558301d61879c27edc48dc4148ca187f