USB: gadget: android: Support serial function driver

Serial function driver now uses newly introduced function
framework which allows it to be independently built and also
support CONFG_FS based compositions.
Android composite driver should now use get_(or put_)function
framework APIs to get the instance and function allocated and
is responsible for adding function to the composition using
usb_add_function which earlier was done by function drivers
only. Currently only ACM and SERIAL are using this framework.
Legacy drivers are still using the old approach.

Change-Id: I035302602629bae32605dd1c67a78f69047b35e8
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>