[android] Unify cache and offline databases

[zugaldia]

Per #4362, we're now unifying the ambient and the offline databases in one location. This PR:

1. Moves them both to a permanent storage folder.
2. Deletes any previous ambient database file.

/cc: @jfirebaugh @1ec5

[1ec5]

Noting for posterity that, with this change, the Android SDK continues to have multiple DefaultFileSource instances, whereas the iOS SDK now maintains only one DefaultFileSource instance as of #4372. As far as I know, the main consequence is that the limit of 20 simultaneous connections is enforced per map view / offline manager rather than per process.

[jfirebaugh]

On the iOS side, #4377 ensures that the beta.1 offline database is preserved. It looks like this PR will not take that measure, since it changes the file name used by OfflineManager from "mbgl-offline.db" to "mbgl-cache.db".

Options:

• Keep the PR as-is, and let people know that they'll need to recreate offline regions in beta.2. (If we do this, we should ensure that "mbgl-offline.db" is removed.)
• Restore "mbgl-offline.db" as the name used by OfflineManager, and change the name here to match.
• Keep using "mbgl-cache.db", but write a migration that moves "mbgl-offline.db" to "mbgl-cache.db".

I like the idea of preserving existing beta.1 offline databases in one of the latter two ways, but I don't feel super strongly about it. What do you think?

[zugaldia]

Restore "mbgl-offline.db" as the name used by OfflineManager, and change the name here to match.

@jfirebaugh Good catch. Tackled by f5b750b.

[jfirebaugh]

Now DATABASE_NAME needs to be replaced with "mbgl-cache.db" here.

[zugaldia]

Yes, indeed: 7fcfc2c.

[jfirebaugh]

👍

Tested and confirmed that an existing offline region is preserved, and then used when there is not network access.