aconfig: fix build issue

When trying to advance the CHECK_API flag, the build failed (so the CL
couldn't submit) with an error that Build is not found. This happened
because for the Flags template, I only gated the import statement on
library_exported, not the SDK check itself. Fixed this and added a test
to make sure SDK check is not added to Flags in internal codegen, even
when the flag is on the finalized list.

Also remove a build import from FeatureFlagsImpl where it was unused.

Change-Id: I6474a67665169c9da1fc6325017eeaa9d5847048
Test: atest aconfig.test