Allow REST fallback via remote config.

app/src/main/java/org/thoughtcrime/securesms/dependencies/ApplicationDependencyProvider.java

@@ -170,7 +170,8 @@ public class ApplicationDependencyProvider implements AppDependencies.Provider {
                                             keysApi,
                                             Optional.of(new SecurityEventListener(context)),
                                             SignalExecutors.newCachedBoundedExecutor("signal-messages", ThreadUtil.PRIORITY_IMPORTANT_BACKGROUND_THREAD, 1, 16, 30),
-                                            ByteUnit.KILOBYTES.toBytes(256));
+                                            ByteUnit.KILOBYTES.toBytes(256),
+                                            RemoteConfig::useMessageSendRestFallback);
   }
 
   @Override

app/src/main/java/org/thoughtcrime/securesms/util/RemoteConfig.kt

@@ -1087,5 +1087,13 @@ object RemoteConfig {
     hotSwappable = false
   )
 
+  @JvmStatic
+  @get:JvmName("useMessageSendRestFallback")
+  val useMessageSendRestFallback: Boolean by remoteBoolean(
+    key = "android.useMessageSendRestFallback",
+    defaultValue = false,
+    hotSwappable = true
+  )
+
   // endregion
 }