Expose getSubId to ensure we have correct subId state in IMS

Expose getSubId in order to allow consumers of ImsManager to
know if ImsManager does not match the requested subId.

Fixes: 218893458
Bug: 219529364
Test: atest ImsCommonTests
Change-Id: Ie9838dde5ce628b13937078fa9acf878a0b71dc4