OTS — ActivityPolicyValues and how they relate to Activity service object

The current specification defines ActivityPolicyValues that were based on
the OTSPolicyValues at a time of flux for the OTS specification. These are
currently:
const ActivityPolicyValue IGNORES = 0; // This is the default
const ActivityPolicyValue SUPPORTS = 1;
const ActivityPolicyValue REQUIRES = 2;
const ActivityPolicyValue REQUIRES_NONE = 3;

Since OTS 1.2 is now finalised, I believe we should follow the pattern
established by that specification and replace these policies with the
following:
const ActivityPolicyValue REQUIRES = 1;
const ActivityPolicyValue FORBIDS = 2;
const ActivityPolicyValue ADAPTS = 3;
I'm not sure which policy should be used as a default in the absence of an
explicit ActivityPolicy; I propose ADAPTS since FORBIDS seems a little
harsh.
The definitions for these policies should follow the equivalent OTSPolicy
definitions.
I don't believe we need any equivalent of the NonTxTargetPolicyValue
(required only for interoperation with OTS 1.1).

We need to define the policy that the Activity service objects themselves
should use to determine whether or not context is expected to flow on
operations on these objects. I believe this should be ADAPTS for all
Activity service objects.

Update the specification in line with the OTS