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 commit 306de26c138ec135846f2a74a952464cf00dce54)
5d0d638f54