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"

Here's what the new notification looks like on a modern Android device, alongside other notifications:

![screenshot_2013-12-22-04-51-59](http://i.imgur.com/N6NdpNE.png)