Stop prefix discovery if an RA prefix arrives in DISCOVERING

Currently, if a prefix is learned from an RA while prefix
discovery is running, clatd will be correctly started, but
prefix discovery will be stopped.

In order to fix this, make it possible to call
stopPrefixDiscovery without transitioning to IDLE state (which
is obviously necessary in this case), by moving the assignment of
the next state from that method to its callers. For consistency,
do the same for startPrefixDiscovery.

Bug: 150648313
Test: new test coverage
Change-Id: I3803fa3d9806848b331c35ee8bac256934bd1f21
Merged-In: I3803fa3d9806848b331c35ee8bac256934bd1f21
(cherry picked from commit c7c6f764)