Disable incorrect strong auth prompt strings.

The prompt strings on the confirm credentials screen (pin, password,
pattern) are incorrect. They currently say strong auth is "required
after device restarts". But instead they should be "required for
additional security" because strong auth can be enforced not only after
device or profile restarts, but also after profile key eviction, for
example.

Unfortunately, we've already missed the window for string changes.
Therefore, as an alternative, we use generic prompt strings in this CL,
to avoid conveying the incorrect (and misleading) information. We'll
follow up with another CL in master with a proper string change to fix
the issue.

Bug: 36511626
Test: manual
Test: make SettingsRoboTests
Change-Id: I44f84420b88bb4933ad0afa6e8032af465de0cd3