Add activity for just clock face picker.

The activity can be started by an intent with action:
  com.google.android.apps.wallpaper.ACTIVITY_CLOCK_FACE_PICKER
When the activity finishes, it sets the ID of the selected
clock into an extra with name "clock_face_name".

Bug: 130021301
Test: Launched activity with `adb shell am start -a "com.google.android.apps.wallpaper.ACTIVITY_CLOCK_FACE_PICKER"`
Change-Id: Iea974d011b786587300564e3aa25f7e85add6fb2
Test: Checked that clock not changed on lock screen/AOD