Add API for deferring fragment start.

Fragments now have the setDeferStart method to signal that a fragment
has lower priority than others. Deferred start fragments will not
always be started immediately; they will be started once any loaders
have finished servicing any outstanding requests. This is useful if
any attached fragments are not immediately visible and can wait to
start until later.

Disabling deferStart on a fragment that is waiting for a deferred
start will start it immediately. Start.

Change-Id: Ia1f004877ca5e88d4f10147d21c7e2e97f141c34