Add generic import support for Codegen

Supports any kind of DataClass or NonNull-type annotations instead of
enforcing the Android framework package.

Bug: 195012034

Test: manual, generate for DomainVerificationInfo, verify no change
Test: manual, generate for mainline class, verify no internal import

Change-Id: I0360281aa20b7f2c6e75dec88a92b810fa69b9b7