Update assertion on DmTargetDefaultKey param string

Due to some recent code updates that introduced some additional
extra_argv, the DmTargetDefaultKey::GetParameterString() test was
failing.
Update the string accordingly.

Bug: none
Test: libdm_test
Change-Id: I63fab27e32a08211a43d4804d90a6f19ea4edc06
Signed-off-by: Alessio Balsini <balsini@google.com>