Soong: Define a new Soong module named prebuilt_firmware.

There are several firmware files that are being installed to
system/etc/firmware or vendor/firmware. A new module type is
defined to install firmware files to install in the system
or vendor image.

Fixes: b/133711895
Test: Ran lunch, wrote unit test cases, created a sample Android.bp
      with defined prebuilt_firmware Soong and ran mma.

Change-Id: Iaa791cdda4402936037c5a456cc8bf8e6c905b3e