Various JobManager performance improvements.

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

@@ -0,0 +1,29 @@
+package org.thoughtcrime.securesms.util.concurrent;
+
+import androidx.annotation.NonNull;
+
+import org.thoughtcrime.securesms.util.Util;
+
+import java.util.concurrent.Executor;
+
+/**
+ * If submitted on the main thread, the task will be executed on the provided background executor.
+ * Otherwise, the task will be run on the calling thread.
+ */
+public final class NonMainThreadExecutor implements Executor {
+
+  private final Executor backgroundExecutor;
+
+  public NonMainThreadExecutor(@NonNull Executor backgroundExecutor) {
+    this.backgroundExecutor = backgroundExecutor;
+  }
+
+  @Override
+  public void execute(@NonNull Runnable runnable) {
+    if (Util.isMainThread()) {
+      backgroundExecutor.execute(runnable);
+    } else {
+      runnable.run();
+    }
+  }
+}