Make Cookes "expires" parsing more lenient

OpenJdk cookies implementation is more
strict about the "expires" field parsing and requires
the "GMT" prefix before timezone value. This is a
regression from Android M and earlier. We were more
lenient and accepted values without the GMT prefix.

This change switches "expires" parsing code to use
libcore.net.http.HttpDate code.

Bug: 31039416
Test: CtsLibcoreTestCases
Change-Id: I13359ed3451fa480ee6f949fd657bf639335ef37