Simplify password length constraints checking

Currently minimum password length policy is queried twice:
1. When constructiong the intent in
   ChooseLockGenericFragment.getIntentForUnlockMethod and then
   passed into setPasswordLengthRange in getLockPasswordIntent
2. in ChooseLockPasswordFragment.processPasswordRequirements via
   LockPatternUtils.getRequestedMinimumPasswordLength().

These two values are then combined in processPasswordRequirements
using Math.max(), which doesn't make sense since it is the same
value.
With this CL it is only queried once in processPasswordRequirements.

+ cleaned up code filling in unused list.
+ removed unused extras, since they are never set anywhere.

Bug: 30558331
Test: atest ChooseLockPasswordTest
Test: atest SetupChooseLockPasswordTest
Test: atest ChooseLockGenericTest
Test: manual, set password policy and change password.

Change-Id: Ifc4946d5b3b26131da01178fa9c827de7a52c7c6