Be bug-compatible with Fragment#setUserVisibleHint < N

Prior to Android N we were simply checking if a fragment had a
FragmentManager set before we would trigger a deferred
start. Unfortunately this also gets set before a fragment transaction
is committed, so if setUserVisibleHint was called before a transaction
commit, we would start the fragment way too
early. FragmentPagerAdapter triggers this situation.

Unfortunately some apps relied on this timing in overrides of
setUserVisibleHint on their own fragments, and expected, however
erroneously, that after a call to super.setUserVisibleHint their
onStart methods had been run.

Bug 28184671

Change-Id: Ie40d5f6963d312c2fad4a48fb4f992d33e65c83b