new intro image for pairing

mailbox-android/artwork/qr_code_scan.svg

mailbox-android/src/main/AndroidManifest.xml

...mailbox/android/keyagreement/MailboxExchangeActivity.java

mailbox-android/src/main/res/drawable/qr_code_scan.xml

...-android/src/main/res/layout/fragment_keyagreement_id.xml

mailbox-android/src/main/res/layout/fragment_setup_intro.xml

mailbox-android/src/main/res/menu/contact_list_actions.xml

mailbox-android/src/main/res/menu/overview_actions.xml

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