[access] return value only when subscribed

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[init] emit valueChanged only if it was really changed

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[access] waiting for subscription can be expensive

e.g. upower can delay loading

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>