USB Manager string clean up:

- Use "Cancel" instead of "Ignore"
- Customize the text for the "always use" checkbox for devices and accessories
- Clean up strings for AccessoryChat test program

Bug: 4074719

Change-Id: Ideec838e4c1f1a82ef4ae411c9124417ffb63165
Signed-off-by: Mike Lockwood <lockwood@android.com>