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

media/mtp/MtpServer.h

@@ -34,8 +34,11 @@ namespace android {
 
 class IMtpDatabase;
 class MtpStorage;
+class MtpMockServer;
 
 class MtpServer {
+    // libFuzzer testing
+    friend class MtpMockServer;
 
 private:
     IMtpDatabase*       mDatabase;