Implement reset to original selection.

The UX for smart selection specifies that the user is able to reset
the original selection by tapping once on the word after smart
selection. Tap once, and the selection resets to the word that was
initially selected (before smart selection happened), tap one more
time and the insertion cursor appears as usual. If the user taps
a different word other than the original selection, the insertion
cursor appears as before.

Test: I7456eb4773d40366a3f4aa7bf051a1c7ddda6e9d
Bug: 34777048
Change-Id: If73259ddb67379d5a5deaa01b840b5185cedf4c8