check-flagged-apis: add new CLI to verify @FlaggedApi APIs

Add a new CLI to verify that the build artifacts contain the right set
of @FlaggedApi APIs, based on the value of the corresponding aconfig
flag.

This CLI will act as an end-to-end test of Soong and metalava.

This CL only adds the project scaffolding; later CLs will add the
implementation.

Bug: 334870672
Test: m check-flagged-apis && check-flagged-apis
Change-Id: Ib00653f2a549217da2b0058867c711f35efd5760

PREUPLOAD.cfg

@@ -1,2 +1,5 @@
 [Hook Scripts]
 do_not_use_DO_NOT_MERGE = ${REPO_ROOT}/build/soong/scripts/check_do_not_merge.sh ${PREUPLOAD_COMMIT}
+
+[Builtin Hooks]
+ktfmt = true