sensord: fix the problem that attribute is not restored properly

sensord: fix the problem that attribute is not restored properly

if command including "0", e.g., command is "-63, 23, 50, 0, 0, 0, 0", is
sent to sensord, just "-63, 23, 50" command is stored to cache.
because std::string recognize 4th character as NULL termination.
so value and value_len should be stored to cache.

Change-Id: Ib04afa1f9824a679b546ce9c284b91d389172a82
Signed-off-by: kibak.yoon <kibak.yoon@samsung.com>