support cellbroadcast data migration by dafault

As part of mainline effort, cellbroadcastreceiver package name has been
renamed. Now its a completely new apk with different uid. That said all
user data e.g, cellbroadcast history and user preference from legacy app
com.android.cellbroadcastreceiver are gone. This change is to support
preserve user data when devices upgrate to R and take cellbraodcast.
mainline module.
1. create legacy cellbroadcast app with old pakcage name
com.android.cellbroadcastreceiver. this app only surface the old data
and should not contains any activities/services to handle emergecy apert
2. legacy cellbroadcast app will be included to the system image by
default. OEMs are free to remove it if they don't care data loss or
after R data migration is done. leagcy app will not be part of
com.android.cellbroadcast apex.
3. the real mainline module rename to com.android.cellbroadcast.module

Bug: 155844209

Change-Id: I5e61c7e777526e038cd8d9971a2c5b87c00eaacb
Merged-in: I5e61c7e777526e038cd8d9971a2c5b87c00eaacb