Apps: Refactor applications verification logic

The new logic uses:

- Fdroid API to verify if an application exists on it and verifies it using the signature
- Introduces a new json file to store default system applications and verifies it using their signature
- Verifies non-fdroid and non-system applications using sha1sum

Signed-off-by: Aayush Gupta <theimpulson@e.email>
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