[Feature sync] Resolve class-level nullness suppression

This is a chery-picked commit from internal branch which is to
add nullability to reslove the nullness suppression.

Bug: 242631897
Bug: 254155029
Test: atest FrameworksNetTests
Change-Id: I289b0408c0c4ccdefc498d3b6d3f381c2b04132e