Add flag and API to enable/disable Slow keys

DD: go/pk_accessibility
‘Slow keys’ is an accessibility feature to aid users who have
physical disabilities, that allows the user to specify the
duration for which one must press-and-hold a key before the
system accepts the keypress.

Bug: 294546335
Test: atest --host libinputflinger_rs_test
Change-Id: I88e518c050f14451560773e444943e33bab09f0c