Mark profile password key as critical.

Currently it is erased when keystore for user 0 is cleared, e.g.
when the user clears data for Settings app.

Bug: 64467610
Bug: 35929605
Test: Manually cleared Settings data.
Test: Manually cleared credentials from Settings.
Change-Id: I7e8753a1bf53f5d68d4738a4eb84faa890f026cc