check-flagged-apis: extract argument names into constants

Extract the command line argument names (and help texts) into constants.
This will allow future subcommands to re-use the same names and keep
things consistent.

Bug: 345207706
Test: atest check-flagged-apis-test
Flag: EXEMPT host side tool
Change-Id: I430f36c99f28aab8511a357f572086ee238d653b