usb: gadget: android: Add FunctionFS

Add support for FunctionFS (ffs) to implement usb functions in userspace.

The aliases property stores the list of functions that are implemented
using functionfs.

For example:
echo "adb,mtp" > /sys/class/android_usb/android0/f_ffs/aliases

Then when the function are enabled:
echo "adb,acm" > /sys/class/android_usb/android0/functions
Internally, ffs and acm will be used.

Change-Id: I44117b183d48a5a99ddbee3ef2cf8998be74598e
Signed-off-by: Benoit Goby <benoit@android.com>