This change amends the doRestore() / onRestore() interface to backup agents to
provide the integer android:versionCode of the app that stored the backup set.
This should help agents figure out how to handle whatever historical data set
they're handed at restore time.
Backups will be handled by launching the application in a special
mode under which no activities or services will be started, only
the BackupAgent subclass named in the app's android:backupAgent
manifest property. This takes the place of the BackupService class
used earlier during development.
In the cases of *full* backup or restore, an application that does
not supply its own BackupAgent will be launched in a restricted
manner; in particular, it will be using the default Application
class rather than any manifest-declared one. This ensures that the
app is not running any code that may try to manipulate its data
while the backup system reads/writes its data set.
