Prevent duplicate loader onLoadFinished calls on config change

Loaders report entering the started state in two places, once from
their host callbacks and once when moving into their host fragment's
starting state. In the former, we will also deliver load results if
we're finishing a retained cycle.

In practice, the individual fragment start happens first which clears
the report-next-start flag, then the finishRetain step sees that flag
is cleared and dispatches the finished load results again. Change
reportStart to only call onLoadFinished if we are not finishing up a
retain step.

Bug 28074512

Change-Id: I85b848f7d7b239c8fac5aec8f0c91e4eea6bcebf