Create random avatar for test contacts

This MR factors out update message encoding from the AvatarManager into an AvatarMessageEncoder, so that it can be used to create messages for testing. Then it introduces a simple TestAvatarCreator interface and implements this only for Android debug builds. This is then used by the TestDataCreator to add avatars to test contacts. Finally, the last commit allows the percentage of contacts with avatars to be configured in the UI.

Dev part of #1848 (closed)