check-flagged-apis: add support for subcommands

The tool currently only supports a single subcommand, the "check"
command. Follow-up CLs will add new subcommands.

Bug: 345207706
Test: build/tools/check-flagged-apis/check-flagged-apis.sh
Flag: EXEMPT host side tool
Change-Id: I1aaaf313db8a10a7427aab378aac8d946d5a8a3d