[SM09] Add helper class to monitor RAT type change per sub

In current design, Rat type change in NSS is only for default
subscription. This is only correct for single sim project.
However, it does not correct for multi-sim scenarios such as
CBRS or DSDS, given that all data usage will be attributed
to the Rat type of default sub.

Thus, add a helper class to monitor subscription change event,
and register/unregister dynamically for Rat type change for
every subscription.

Note that unit test of the helper class will be addressed
in follow-up patch.

Test: m -j
Bug: 146415925
Change-Id: I0055f6b55c209a073ce997fc7a144477f9db7069
Merged-In: I0055f6b55c209a073ce997fc7a144477f9db7069
(cherry picked from ag/10977156 and fixed conflict)