Allow opening account settings from the Accounts list in the Settings app.

* app/src/main/AndroidManifest.xml: SettingActivity: Specify an
  <intent-filter> so this activity can be opened via an (arbitrary, I think)
  action:name.
* app/src/main/res/xml/account_preferences.xml: A new <PreferenceScreen>
  (actually one inside another, as is apparently necessary) to show
  in the Settings app under the account.
  This has a child <intent> which will open our SettingActvity (with the
  action:name form AndroidManifest.xml.
  We could instead show preferences directly but that is a) harder,
  and b) maybe confusing.
* app/src/main/res/xml/authenticator.xml: Specify account_preferences.xml
  with android:accountPreferences so it shows up in the Settings app.