VmsUtils: Update existing methods to parse subscriptions response messages.

Currently, we do not have a way to parse subscriptions response
messages. But we have parsing methods for SUBSCRIPTIONS_CHANGE type of
messages. These messages have the exact same payload as SUBSCRIPTIONS_RESPONSE
messages. So all we need to do is allow existing methods to parse
SUBSCRIPTIONS_RESPONSE messages along with SUBSCRIPTIONS_CHANGE
messages.

Bug: 138664881
Fixes: 138664881, 137955333
Test: Added more tests for the new message type. Ran tests on Hawk.
Change-Id: I53432e2215e03bb94fd36b7d62f42fe0be1994cf