Do not backup the messages db

[RankoR]

bugle_db ties each conversation to an SMS thread ID from the phone's SMS database. When SMS are restored, that database gives the threads new IDs, and this causes wrong thread/contact linking.

[inthewaves]

There are likely other places that use bugle_db specific IDs that are being backed up

e.g. https://github.com/GrapheneOS/Messaging/blob/main/src/com/android/messaging/widget/WidgetConversationPrefs.kt uses the conversations._id autoincrementing key as a string in bugle_widgets.xml shared preferences, so if bugle_db is no longer backed up, the internal autoincrement id wouldn't make sense on new installs

Notification channels in https://github.com/GrapheneOS/Messaging/blob/main/src/com/android/messaging/util/NotificationChannelUtil.kt also use the internal autoincrement id, and Android backs up those notification settings + channels separately from the app backup (https://github.com/GrapheneOS/platform_frameworks_base/blob/17/services/core/java/com/android/server/notification/PreferencesHelper.java#L830)