aconfig: Add API level check to exported mode.

API level check means that for exported flags guarding a finalized API,
we consider the flag always true on Android builds where the API is
finalized (so, the SDK version is high enough). In other words, we check
the SDK level in the flag accessor compared against the SDK level in
which the API the flag was guarding is finalized. If it's high enough,
we return true without actually performing the flag lookup because the
guarantee of finalization is that this API will now always be available
(so the flag should always be true). This is more performant.

Changes:
-add flag to aconfig binary (sdk-check) to toggle the API level check in
codegen, default to false and can be toggled via build flag
-pipe bool through to the codegen
-add api level check in new exported codegen

Next steps:
-add build flag
-generate proto <flag, API level> map from text list of finalized flags
-read artifact to set finalized_sdk_value in FlagElement

Flag: see above, guarding from binary, build flag TODO
Test: atest aconfig.test
Change-Id: I9bd4ee3e4d005acb812bd4010dd5cb301fa8de58