Separate the SelectionActionMode into Insertion and Selection.

When we got rid of the paste popup, we merged it into the Selection
ActionMode and moved all its invocations to the ActionMode. Some apps
actually want the paste popup without the Selection ActionMode, hence
separating them again allows them to cancel the one they want.

Bug: 21571422
Change-Id: I91bcd0d9c3e68d9c736698fe0bec010b4c9f5cf3