binding: data-persistence: Modified the response of verb_read

binding: data-persistence: Modified the response of verb_read

Added key and value both in response so that user can know which key the value corresponds to.
The current persistence implementation responds with only the value when queried. Since this is
async call and multiple keys can be queried at the same time, the key and value should be given
as response to any persistence query done. This will improve the readability of the Read verb
response for user application.

Also made the same changes for failed case of Read verb so that if read verb fails, user can know
for which key it failed.

Bug-AGL: SPEC-2828

Signed-off-by: Gaykawadpk <Puja.Kumari@kpit.com>
Change-Id: I565cd78ca2c1e7c5d5f63ab49479b495044772ce