Prevent null pointer exceptions

The main activity connects to the service after setting up the local
broadcast. This allows the service to send notification before the
activity is ready, causing null pointer exceptions. This happens for
example rotating the screen while an update is being installed.

Also, replace the existing check on mUpdaterController with a check
on mDownloadsId since the latter depends on the former and both are
required.

Change-Id: I620fd7aa1e90468ab40dfedaa06c23f96f3e6807