Merge branch '1224-show-explanation-when-contact-exchange-fails' into 'master'

...iarproject/bramble/keyagreement/KeyAgreementTaskImpl.java

briar-android/artwork/qr_code_error.svg

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

...t/briar/android/keyagreement/ContactExchangeActivity.java

...ar/android/keyagreement/ContactExchangeErrorFragment.java

...ject/briar/android/keyagreement/KeyAgreementActivity.java

...ject/briar/android/keyagreement/KeyAgreementFragment.java

...briarproject/briar/android/settings/SettingsFragment.java

...ain/java/org/briarproject/briar/android/util/UiUtils.java

briar-android/src/main/res/drawable/qr_code_error.xml

...d/src/main/res/layout/fragment_error_contact_exchange.xml

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