Add activity variables and late context variable change.

Activity variables are like context-rules, but set when sink is in
active or inactive state. Currently supports only set-property style
actions, defined in 'active' and 'inactive' fields. For example

[activity]
device       = headphone
sink-name    = equals:sink.hw0
active       = sink-name@equals:"sink.null", property:"some.property", value@constant:"true"
inactive     = sink-name@equals:"sink.null", property:"some.property", value@constant:"false"

Also change context variable setting to happen last when parsing policy
actions, using pa_policy_context_variable_commit().