Added a Singleton VaultHolder with weak references to Vaults

This concept has to be reviewed!!

It should allow the app to share a Vault over different views without re-creating the vault with the expensive PBKDF2 operation.

The performance of the app (after the first load) is now exactly like with the old ECB Vaults.

But it is crucial, that the VaultHolder is cleared when leaving a Vault (in every possible way (Home button, back button, whatever) otherwise a Vault might still be accessible without the correct password. I would like to have input if the .clear() method is really called in every situation.