Add a system to improve app foreground observation.

There was previously a crash that occurred when multiple threads tried to use ProcessLifecycleOwner, and this will hopefully resolve that.

app/src/main/java/org/thoughtcrime/securesms/util/AppForegroundObserver.java

@@ -0,0 +1,90 @@
+package org.thoughtcrime.securesms.util;
+
+import androidx.annotation.AnyThread;
+import androidx.annotation.MainThread;
+import androidx.annotation.NonNull;
+import androidx.lifecycle.DefaultLifecycleObserver;
+import androidx.lifecycle.LifecycleOwner;
+import androidx.lifecycle.ProcessLifecycleOwner;
+
+import java.util.Set;
+import java.util.concurrent.CopyOnWriteArraySet;
+
+/**
+ * A wrapper around {@link ProcessLifecycleOwner} that allows for safely adding/removing observers
+ * on multiple threads.
+ */
+public final class AppForegroundObserver {
+
+  private final Set<Listener> listeners = new CopyOnWriteArraySet<>();
+
+  private volatile Boolean isForegrounded = null;
+
+  @MainThread
+  public void begin() {
+    Util.assertMainThread();
+
+    ProcessLifecycleOwner.get().getLifecycle().addObserver(new DefaultLifecycleObserver() {
+      @Override
+      public void onStart(@NonNull LifecycleOwner owner) {
+        onForeground();
+      }
+
+      @Override
+      public void onStop(@NonNull LifecycleOwner owner) {
+        onBackground();
+      }
+    });
+  }
+
+  /**
+   * Adds a listener to be notified of when the app moves between the background and the foreground.
+   * To mimic the behavior of subscribing to {@link ProcessLifecycleOwner}, this listener will be
+   * immediately notified of the foreground state if we've experienced a foreground/background event
+   * already.
+   */
+  @AnyThread
+  public void addListener(@NonNull Listener listener) {
+    listeners.add(listener);
+
+    if (isForegrounded != null) {
+      if (isForegrounded) {
+        listener.onForeground();
+      } else {
+        listener.onBackground();
+      }
+    }
+  }
+
+  @AnyThread
+  public void removeListener(@NonNull Listener listener) {
+    listeners.remove(listener);
+  }
+
+  public boolean isForegrounded() {
+    return isForegrounded;
+  }
+
+  @MainThread
+  private void onForeground() {
+    isForegrounded = true;
+
+    for (Listener listener : listeners) {
+      listener.onForeground();
+    }
+  }
+
+  @MainThread
+  private void onBackground() {
+    isForegrounded = false;
+
+    for (Listener listener : listeners) {
+      listener.onBackground();
+    }
+  }
+
+  public interface Listener {
+    void onForeground();
+    void onBackground();
+  }
+}