mediatranscoding: avoid double-ownership in test

SharedRefBase subclasses have an internal weak_ptr and AIDL code
promotes it internally. No error was seen from this, but because
SharedRefBase is being converted to be based on enable_shared_from_this,
this was causing compilation errors.

Bug: 194905504
Test: ./build_and_run_all_unit_tests.sh had some cases fail before and
  it fails now
Change-Id: I3dd5ad0feb9ed020073808c32fa1c5689a29e1b2