Implement Luhn checksum algo

Ignore all non-digits. If only digits should be allowed, the
developer can use a combination of SimpleRegexValidator and
RequiredValidators.

Bug: 62534917
Test: cts-tradefed run cts-dev -m CtsAutoFillServiceTestCases
          --test=android.autofillservice.cts.LuhnChecksumValidatorTest

Change-Id: I4a7c68a236c3d4cb967f1bc61115df89e584a86d