refactor: QrCodeFragment

This commit adds the ability to customize QrCodeFragment with a mode parameter:

    MODE_SCAN: Scan a peer QR code and navigate to the conversation.
    MODE_SHARE: Share a contact's QR code.

Both `MODE_SCAN` and `MODE_SHARE` can be used simultaneously.

ShareFragment has also been reworked. Previously, it could only share the current account; now, it accepts a more general contact parameter.

Additionally, all ViewModel components involved in this rework have been replaced with Presenter implementations for a more streamlined architecture.

Change-Id: Ifa7a9fc4423bd8be5c8ce5e22d25560816cb44e0