Improve password caching notification UI & UX

On Jelly Bean and above:

- Use the standard notification style for a better and consistent visual
  appearance

- Use the JB notification actions API for the locking action

- Use a lower notification priority to prioritize other notifications
  over TextSecure

On ICS:

- Use the existing custom notification layout

Everywhere:

- Allow opening the app itself from the notification

- Simplify strings: don't talk about a "cached passphrase" but about the
  app being "unlocked"/"locked"

res/layout/key_caching_notification.xml

@@ -28,7 +28,7 @@
         android:singleLine="true"
         android:fadingEdge="horizontal"
         android:ellipsize="marquee"
-        android:text="@string/KeyCachingService_textsecure_passphrase_cached"
+        android:text="@string/KeyCachingService_textsecure_passphrase_cached_with_lock"
         />
     </LinearLayout>