Add limit for the bell (vibration)

This fixes https://github.com/termux/termux-app/issues/442

app/src/main/java/com/termux/app/BellUtil.java

@@ -0,0 +1,63 @@
+package com.termux.app;
+
+import android.content.Context;
+import android.os.Handler;
+import android.os.Looper;
+import android.os.SystemClock;
+import android.os.Vibrator;
+
+public class BellUtil {
+    private static BellUtil instance = null;
+    private static final Object lock = new Object();
+
+    public static BellUtil with(Context context) {
+        if (instance == null) {
+            synchronized (lock) {
+                if (instance == null) {
+                    instance = new BellUtil((Vibrator) context.getApplicationContext().getSystemService(Context.VIBRATOR_SERVICE));
+                }
+            }
+        }
+
+        return instance;
+    }
+
+    private static final long DURATION = 50;
+    private static final long MIN_PAUSE = 3 * DURATION;
+
+    private final Handler handler = new Handler(Looper.getMainLooper());
+    private long lastBell = 0;
+    private final Runnable bellRunnable;
+
+    private BellUtil(final Vibrator vibrator) {
+        bellRunnable = new Runnable() {
+            @Override
+            public void run() {
+                if (vibrator != null) {
+                    vibrator.vibrate(DURATION);
+                }
+            }
+        };
+    }
+
+    public synchronized void doBell() {
+        long now = now();
+        long timeSinceLastBell = now - lastBell;
+
+        if (timeSinceLastBell < 0) {
+            // there is a next bell pending; don't schedule another one
+        } else if (timeSinceLastBell < MIN_PAUSE) {
+            // there was a bell recently, scheudle the next one
+            handler.postDelayed(bellRunnable, MIN_PAUSE - timeSinceLastBell);
+            lastBell = lastBell + MIN_PAUSE;
+        } else {
+            // the last bell was long ago, do it now
+            bellRunnable.run();
+            lastBell = now;
+        }
+    }
+
+    private long now() {
+        return SystemClock.uptimeMillis();
+    }
+}

app/src/main/java/com/termux/app/TermuxActivity.java

@@ -400,7 +400,7 @@ public final class TermuxActivity extends Activity implements ServiceConnection
                         mBellSoundPool.play(mBellSoundId, 1.f, 1.f, 1, 0, 1.f);
                         break;
                     case TermuxPreferences.BELL_VIBRATE:
-                        ((Vibrator) getSystemService(VIBRATOR_SERVICE)).vibrate(50);
+                        BellUtil.with(TermuxActivity.this).doBell();
                         break;
                     case TermuxPreferences.BELL_IGNORE:
                         // Ignore the bell character.