Use constants for http property names

Use constants for http property names

Modified the PAP REST server to use constants from policy-endpoints
instead of hard-coding property names.
Also made "alive" volatile.
Also modified to use standard gson as the serialization provider to
ensure that it will always work with gson.
Removed unneeded constant.
Removed trailing whitespace.
Replaced another literal with a constant name.
Updated licenses.

Change-Id: Ibfc6d19ad4dae62ebd610d6127d1d253175ae71e
Issue-ID: POLICY-1444
Signed-off-by: Jim Hahn <jrh3@att.com>