Add README for media "automated" tests

MediaFrameworkTest/

    Uses instrumentation and so can be run with runtest.

    It assumes /sdcard/media_api/ has been populated.

contents/media_api/

    Push to /sdcard/media_api/ for use with MediaFrameworkTest:

    adb shell mkdir /sdcard/media_api

    adb push contents/media_api/ /sdcard/media_api/

All other subdirectories are manual tests or sample apps.