Enforce minimum pattern length in PasswordMetrics

For patterns, currently PasswordMetrics#validateCredential() does
nothing except validate that the pattern credential type is allowed.
Pattern length is supposed to be >= MIN_LOCK_PATTERN_SIZE, but that is
hardcoded into the ChooseLockPattern activity in Settings.

Since there are other places that can set a pattern lockscreen
credential, such as LockSettingsShellCommand, let's add the pattern
length validation to PasswordMetrics#validatePasswordMetrics() so that
it gets done in the same place as PIN and password validation.

Note that this required starting to populate the length field of
PasswordMetrics created for patterns.

Bug: 219511761
Bug: 232900169
Bug: 243881358
Test: atest PasswordMetricsTest
Change-Id: I9f33f86045ea5f674b8ab79f9fda62c988436c53