MTP: add friend class for fuzz testing

In preparation for fuzz testing the MtpServer, we need to be able to
mock the IMtpHandle interface to avoid reading directly from the USB
interface.

Test: SANITIZE_TARGET='hwaddress fuzzer' mmm -j .
Test: atest mtp_ffs_handle_test
Change-Id: I79d023cbda75d6760bed22b659fa31328e9d81c5
1 file changed