Load additional repositories from the custom partition

This feature is the one discussed in #1494 (closed)

This adds additional repositories from the /oem partition on first start-up of the F-Droid Client. More precisely, this adds them whenever the database of F-Droid repos is also created in DBHelper.java. You can see in the diff that it's only a couple imports, an additional string constant and two additional simple functions (one to read a file content into a string and one to extract repos from that string).

The repos are encoded in JSON format. This can be easily extended in the future to XML or any other format anyone likes. For now, it might look as follows:

[
  {
    "name": "Example Repo",
    "address": "https://example.com/repo",
    "description": "This is an example repo to be added on first F-Droid client start-up.",
    "version": "1",
    "enabled": "1",
    "priority": "15",
    "pushRequests": "ignore",
    "pubkey": "FFFFFFFFFFFFFFFFFFFFFFFF...FFFFFFFFFFFFFFFFFFFFFFFF"
  }
]