Add Layer of Encryption to all Local Jami Data

• Ring version: 2019-08-24-01
• Android version: 8.1 / LineageOS 15.1-20190228
• Jami-Version from F-droid

Android is known to have (had) a serious disk encryption flaw: https://techcrunch.com/2016/07/07/android-full-disk-encryption/ With many unpatched Android devices in the wild the current Jami version puts its user at risk.

• Actual result: Android devices opened up by attackers (e.g. border authorities) can betray Jami users because all local Jami data at rest is stored in quasi-plaintext.
• Expected result: All locally stored Jami data shall be protected by an extra layer of encryption.

This should be fixed to prevent similar risk in the future.