Remove the AppMode global

[ignoramous]

Per Daniel,

• VpnController has a static field braveVpnService. This is not an important fix because it is set to null with stop(). If the service is killed by the system it is pulled from memory entirely, so no reason to do cleanup.
• HomeScreenActivity leaks AppMode, which references Context and multiple DAOs. It is used extensively across the app. It is fixable, but I'm not sure whether I'd mess up the logic behind it.
• GoVpnAdapter has a non-static AsyncTask. It potentially leaks Context through this as it is an inner-class. It also is fixable, but requires refactor (also, IntelliSense doesn't work in this class for me).

#188 (comment)

[hussainmohd-a]

Remove some of the inconsistent methods from the Persistentstate.
eg., - modifyAllowedWifi(), getConnectedDNS(), getBraveMode(), setBraveMode()

[ignoramous]

https://github.com/celzero/rethink-app/pull/282/files#r645294634

[ignoramous]

1. VpnController is likely to remain as-is, as it is extensively used throughout the code-base and doesn't seem to critically leak anything, as of yet.
2. AppMode is still a global, but it kind of acts like a DatabaseManager at this point, though not quite. In a limbo, but the code has since been extensively refactored with Connection monitor changes #282
3. GoVpnAdapter's use of AsyncTask has been replaced with Kotlin Coroutines. Replace AsyncTask with Workers #240