Post a notification from the shell.

    $ adb shell cmd notification post --help
    usage: post [flags] <tag> <text>

    flags:
      -h|--help
      -v|--verbose
      -t|--title <text>
      -i|--icon <iconspec>
      -I|--large-icon <iconspec>
      -S|--style <style> [styleargs]
      -c|--content-intent <intentspec>

    styles: (default none)
      bigtext
      bigpicture --picture <iconspec>
      inbox --line <text> --line <text> ...
      messaging --conversation <title> --message <who>:<text> ...
      media

    an <iconspec> is one of
      file:///data/local/tmp/<img.png>
      content://<provider>/<path>
      @[<package>:]drawable/<img>
      data:base64,<B64DATA==>

    an <intentspec> is (broadcast|service|activity) <args>
      <args> are as described in `am start`

Test: atest services/tests/uiservicestests/src/com/android/server/notification/NotificationShellCmdTest.java
Change-Id: I68642b534b5a17a9ba406721fc1860879a6a0e8d