Add feature flag for self-destructing messages.

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

@@ -9,4 +9,5 @@ public interface FeatureFlags {
 
 	boolean shouldEnableProfilePictures();
 
+	boolean shouldEnableDisappearingMessages();
 }

bramble-core/src/test/java/org/briarproject/bramble/test/BrambleCoreIntegrationTestModule.java

@@ -33,6 +33,11 @@ public class BrambleCoreIntegrationTestModule {
 			public boolean shouldEnableProfilePictures() {
 				return true;
 			}
+
+			@Override
+			public boolean shouldEnableDisappearingMessages() {
+				return true;
+			}
 		};
 	}
 }

briar-android/src/main/java/org/briarproject/briar/android/AppModule.java

@@ -283,6 +283,11 @@ public class AppModule {
 			public boolean shouldEnableProfilePictures() {
 				return IS_DEBUG_BUILD;
 			}
+
+			@Override
+			public boolean shouldEnableDisappearingMessages() {
+				return IS_DEBUG_BUILD;
+			}
 		};
 	}
 }

briar-headless/src/main/java/org/briarproject/briar/headless/HeadlessModule.kt

@@ -93,5 +93,6 @@ internal class HeadlessModule(private val appDir: File) {
     internal fun provideFeatureFlags() = object : FeatureFlags {
         override fun shouldEnableImageAttachments() = false
         override fun shouldEnableProfilePictures() = false
+        override fun shouldEnableDisappearingMessages() = false
     }
 }

briar-headless/src/test/java/org/briarproject/briar/headless/HeadlessTestModule.kt

@@ -81,6 +81,7 @@ internal class HeadlessTestModule(private val appDir: File) {
     internal fun provideFeatureFlags() = object : FeatureFlags {
         override fun shouldEnableImageAttachments() = false
         override fun shouldEnableProfilePictures() = false
+        override fun shouldEnableDisappearingMessages() = false
     }
 
     @Provides