Update UDFPS "press to unlock" strings

To make it clear to the user that we're asking
that they press on the unlock icon.

Fixes: 227501872
Test: trigger face auth, see message
Test: enable a trust agent, then tap empty space on LS,
see message

Change-Id: I4f0d5c9e3be2870a038276944e3e2b3e9ae77630