Convert NotificationSectionsManager to Kotlin

This is a direct conversion, split into a separate CL in order to
improve readability of the later CL(s) in this relation chain. There
are no functional changes to the code.

Bug: 153554168
Test: builds, atest
Change-Id: Ic476fbb09d159a17cb811a49002b28b4fa823a57
13 files changed