Moved DomainNameChecker to android common.

- Moved DomainNameChecker from android.net.http to android common, and renamed to DomainNameValidator.
- Added a simplified version of DNParser, which DomainNameValidator uses instead of X509Name in order to extract Subject Name from a certificate.
- Added unit tests for DomainNameChecker and DNParser.

There's a suspicious comment in DomainNameChecker saying something like "X509Certificate fails to parse a certificate when a subject alt name begins with '*'".  I think we should fix it if it's really the case -- otherwise certificates with the wildcard wouldn't work.  I'll see if it's true after submitting this patch.