Add contact exchange to Briar

briar-android/src/main/AndroidManifest.xml

...java/org/briarproject/briar/android/AndroidComponent.java

...riarproject/briar/android/activity/ActivityComponent.java

...t/briar/android/keyagreement/MailboxExchangeActivity.java

...briarproject/briar/android/repeater/RepeaterActivity.java

briar-android/src/main/res/layout/activity_repeater.xml

briar-android/src/main/res/layout/repeater_avatar_status.xml

briar-android/src/main/res/values/strings.xml

briar-android/src/main/res/xml/settings.xml