[SM02] Support record mobile network stats by collapsed rat type

Previously network stats could be recorded by different rat type.
However, the feature was disabled by ag/173504 since rat type
frequently flapping between HSPA+ and UMTS.

Given that this feature might be useful for collecting metrics,
re-implement it based on current architecture and reduce the
overhead introduced by frequently flapping by:
  1. only react when rat type changes between 2G/3G/4G/5G.
  2. reduce the number of records by only recording a subset
     of rat type that represented for a given network class.
  3. enforce 1 second rate limit if flapping too much.

Note that the feature is still disabled but will be enabled
in follow-up patches.

Test: manual test
Bug: 129082217
Change-Id: Ic6b2f10f2c8b082820e0662eb9cee70d70d28cd6