Address review feedback for Connect via Bluetooth UI

.../main/java/org/briarproject/bramble/api/FeatureFlags.java

...roject/bramble/test/BrambleCoreIntegrationTestModule.java

...c/main/java/org/briarproject/briar/android/AppModule.java

...roject/briar/android/conversation/BluetoothConnecter.java

...ndroid/conversation/BluetoothConnecterDialogFragment.java

...ject/briar/android/conversation/ConversationActivity.java

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

...in/java/org/briarproject/briar/headless/HeadlessModule.kt

...ava/org/briarproject/briar/headless/HeadlessTestModule.kt