Allow management of private key and/or identity

Right now it's difficult to demonstrate ownership of a key and the associated signed files. One can take new pictures and show that those are signed with the same key as any photo requiring proof of ownership, but if the app has to be reinstalled or a phone is lost then there is no backup of the secret and it is essentially impossible to prove who produced the evidence.

A simple improvement is to give the user the option to set or modify the `keyId` string to something unique and identifiable.

https://github.com/guardianproject/proofmode/blob/39b6a9e5c7b2ddd627f39b25548c24b73cecdd21/android-libproofmode/src/main/java/org/witness/proofmode/crypto/PgpUtils.java#L78

This probably also requires allowing the user to generate and swap between multiple keys if there is a need to generate both anonymous and attributable evidence.

A more robust approach is to allow users to manage the private key. Being able to import an existing key as in #40 would be ideal. Exporting the private key or an encrypted backup would also be minimal solutions to the problem.