Fix hidl_string copy in boot VTS test

hidl_strings returned through a callback need to be copied; the
current approach just saves a pointer to the underlying buffer, which
is incorrect and sometimes causes the test to fail.

Test: the GetSuffix test now passes reliably
Change-Id: I4bb143be98b00b20f9da4aebef3b5ab4980b7690
Signed-off-by: Connor O'Brien <connoro@google.com>