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