Enhance test for database upgrade process

Global crypto provider

Make Hex.encodeHex returns a String (was char[])

MessageProvider related fixes and testing

Allow any unicode letter to act as a placeholder for contacts with no photo.

Fix consecutive retrying of crypto operations

Exclude NULL from the list of used parts when cleaning up unused ones.

Address: Handle getHostname when mAddress is null

Update robolectric, use Runner class instead of annotations

Remove trailing new lines from subject

Avoid static reference to AttachmentInfoExtractor

Rework ClipboardManager to avoid leaking Context

Fix two unparsable Javadoc comments

Fix a few Javadoc typos

This avoids holding onto a Context instance in a static field.

Use json for serializing pending commands

Add encrypted.asc filename to PGP/MIME emails

SliderPreference - Fix lint warning

Fix notification actions

Test and fix database upgrade process

Add additional debug to diagnose message sync issues

Move hardcoded text to string resources

Make base activities abstract

Tests for EmailHelper

Tests for OpenPgpApiHelper

Basic behavioural tests of the EmailProviderCache

Code refactoring in preparation of HtmlCleaner replacement

Don't load all local message metadata into memory during synchronize

Better determination of whether a draft needs saving

Handle connection closing after authentication failure

Address file clean-up

Fixes #2134

Clean up some of the top Checkstyle offenders