ANDROID: gnss: Add command line test driver

This driver enables the "takeover" of a serdev bus device by the GNSS
subsystem. It can be used to test the GNSS subsystem without needing to
write a specific hardware backend.

The new module supports the following parameters:

  gnss_cmdline.serdev=driver/port/serdev
  gnss_cmdline.type=<protocol number>

An example which allows GNSS to wrap a platform serial8250 port and
advertise NMEA-0183 data is:

  gnss_cmdline.serdev=serial8250/serial0/serial0-0
                        ^          ^       ^
                       driver     port    serdev
  gnss_cmdline.type=0
                    ^
                    GNSS_TYPE_NMEA

Bug: 146517987
Change-Id: I421386ee4f2ba8f1f0832d9c56a067a600892d3c
Signed-off-by: Alistair Delva <adelva@google.com>