Fix a bug where we could lose a loader content change.

If AsyncTaskLoader starts a background update due to a
content change, and that update is cancelled, we drop the
data when it finally arrives and forget that the content changed.
If we later come back to the loader, we then end up showing
stale data because we don't know that we still need to update
due to the old content change.

This change adds a couple new APIs to Loader to deal with the
time between when you ask for whether there is a content change
and finally either commit the data or cancel the update.
AsyncTaskLoader is changed to make use of this so that it doesn't
lose changes.

Change-Id: I3866236b1c22bb9138f2d9f6032b126aeaee2e6e