Fix parole scheduling bugs.

There were a few subtle bugs in app idle parole scheduling that
would cause us to never end a parole period we had entered.  The
updated logic in this change concentrates the registration for the
next event in one place in setAppIdleParoled().

Bug: 24050462
Change-Id: I1efe43cd6e00a547f70c4a4a37e1f3ef52a7e706