Don't double prompt on booting encrypted device

vold will store password securely until KeyGuard requests it
and hands it on to KeyStore.

This is a revision of

https://googleplex-android-review.git.corp.google.com/#/c/418123/

which was reverted. It had two bugs in LockSettingsService.checkVoldPassword.
1) We were not checking password for null, which caused an exception
2) checkPattern/checkPassword return true if there is no saved pattern or password.
   This leads to situations where we get true returned even when the password
   doesn't match. Call the correct one based on what is there, not what vold
   thinks ought to be there.

Bug: 12990752
Change-Id: I05315753387b1e508de5aa79b5a68ad7315791d4