Fixed log warning for calling doStart() twice.

Bug 36504927

The LoaderManager was performing a doStart() multiple times
and issued a warning and then returned without any problems.
This CL fixes the underlying problem of doStart() being
called multiple times so that the warning isn't logged.

Test: ran fragment cts tests
Change-Id: I4a43280fd581e08ccd911d8d05367e00382d694a