check-flagged-apis: add list subcommand

Add a new subcommand to list all flagged APIs and corresponding flags.
This provides an overview of what flagged APIs exist in the Android
tree.

Bug: 345207706
Test: build/tools/check-flagged-apis/check-flagged-apis.sh list
Test: atest check-flagged-apis-test
Flag: EXEMPT host side tool
Change-Id: Icc224f3787480353baabbd3946f36f003f35db59