New pairing flow

mailbox-android/artwork/single_scan_left.svg

mailbox-android/build.gradle

mailbox-android/src/main/AndroidManifest.xml

...arproject/mailbox/android/activity/ActivityComponent.java

...briarproject/mailbox/android/activity/ActivityModule.java

...ct/mailbox/android/keyagreement/KeyAgreementActivity.java

.../mailbox/android/keyagreement/KeyAgreementController.java

...lbox/android/keyagreement/KeyAgreementControllerImpl.java

...ilbox/android/keyagreement/KeyAgreementEventListener.java

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

...ject/mailbox/android/keyagreement/ScanQrCodeFragment.java

...ject/mailbox/android/keyagreement/ShowQrCodeFragment.java

...iarproject/mailbox/android/overview/OverviewFragment.java

...ava/org/briarproject/mailbox/android/view/QrCodeView.java

...riarproject/mailbox/android/viewmodels/BaseViewModel.java

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

...ndroid/src/main/res/layout/fragment_keyagreement_scan.xml

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