Show remaining time on wave form view and cache wave form in database.

app/src/main/java/org/thoughtcrime/securesms/util/concurrent/SerialExecutor.java

@@ -0,0 +1,39 @@
+package org.thoughtcrime.securesms.util.concurrent;
+
+import androidx.annotation.NonNull;
+
+import java.util.ArrayDeque;
+import java.util.Queue;
+import java.util.concurrent.Executor;
+
+/**
+ * From https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Executor.html
+ */
+public final class SerialExecutor implements Executor {
+  private final Queue<Runnable> tasks    = new ArrayDeque<>();
+  private final Executor        executor;
+  private       Runnable        active;
+
+  public SerialExecutor(@NonNull Executor executor) {
+    this.executor = executor;
+  }
+
+  public synchronized void execute(final Runnable r) {
+    tasks.offer(() -> {
+      try {
+        r.run();
+      } finally {
+        scheduleNext();
+      }
+    });
+    if (active == null) {
+      scheduleNext();
+    }
+  }
+
+  private synchronized void scheduleNext() {
+    if ((active = tasks.poll()) != null) {
+      executor.execute(active);
+    }
+  }
+}