Credential FRP: keep gatekeeperd credentials after reset

Gatekeeperd now delays clearing all user credentials
until the device setup is complete or we enroll a new
credential (whichever comes first).

Bug: 36814845
Test: Set lockscreen credential, "adb reboot-bootloader && fastboot -w", "adb shell am start -a android.app.action.CONFIRM_FRP_CREDENTIAL", verify that credential still works
Change-Id: If2ad78ff5b80a6ddffd997be0949b03ed11797f4