added a service to handle the caching, this'll ensure the cache works while no activity is around, which is better for k9mail integration

it also is a more efficient and much smarter cache, not requiring an own timer thread, just a service that sleeps must of the time, it also is more accurate in cleaning up the entries, ensuring that the worst case of too late removal is 5 seconds

src/org/thialfihar/android/apg/Service.java

@@ -0,0 +1,79 @@
+package org.thialfihar.android.apg;
+
+import android.content.Intent;
+import android.os.Binder;
+import android.os.Handler;
+import android.os.IBinder;
+
+public class Service extends android.app.Service {
+    private final IBinder mBinder = new LocalBinder();
+
+    public static final String EXTRA_TTL = "ttl";
+
+    private int mPassPhraseCacheTtl = 15;
+    private Handler mCacheHandler = new Handler();
+    private Runnable mCacheTask = new Runnable() {
+        public void run() {
+            // TODO: I suppose we could read out the time left until the first cache entry
+            // expiration, then use that for the timer...
+
+            // check every ttl/2 seconds, which shouldn't be heavy on the device (even if ttl = 15),
+            // and makes sure the longest a pass phrase survives in the cache is 1.5 * ttl
+            int delay = mPassPhraseCacheTtl * 1000 / 2;
+            // also make sure the delay is not longer than one minute
+            if (delay > 60000) {
+                delay = 60000;
+            }
+
+            delay = Apg.cleanUpCache(mPassPhraseCacheTtl, delay);
+            // don't check too often, even if we were close
+            if (delay < 5000) {
+                delay = 5000;
+            }
+
+            mCacheHandler.postDelayed(this, delay);
+        }
+    };
+
+    static private boolean mIsRunning = false;
+
+    @Override
+    public void onCreate() {
+        super.onCreate();
+
+        mIsRunning = true;
+    }
+
+    @Override
+    public void onDestroy() {
+        super.onDestroy();
+        mIsRunning = false;
+    }
+
+    @Override
+    public void onStart(Intent intent, int startId) {
+        super.onStart(intent, startId);
+
+        mPassPhraseCacheTtl = intent.getIntExtra(EXTRA_TTL, 15);
+        if (mPassPhraseCacheTtl < 15) {
+            mPassPhraseCacheTtl = 15;
+        }
+        mCacheHandler.removeCallbacks(mCacheTask);
+        mCacheHandler.postDelayed(mCacheTask, 1000);
+    }
+
+    static public boolean isRunning() {
+        return mIsRunning;
+    }
+
+    public class LocalBinder extends Binder {
+        Service getService() {
+            return Service.this;
+        }
+    }
+
+    @Override
+    public IBinder onBind(Intent intent) {
+        return mBinder;
+    }
+}