Adjust Uri host parsing to use last instead of first @.

Malformed authority segments can currently cause the parser to produce
a hostname that doesn't match the hostname produced by the WHATWG URL
parsing algorithm* used by browsers, which means that a URL could be seen
as having a "safe" host when checked by an Android app but actually visit
a different host when passed to a browser.  The WHATWG URL parsing
algorithm always produces a hostname based on the last @ in the authority
segment, so we do the same.

* https://url.spec.whatwg.org/#authority-state resets the "buffer", which
  is being used to build up the host name, each time an @ is found, so it
  has the effect of using the content between the final @ and the end
  of the authority section as the hostname.

Bug: 68341964
Test: vogar android.net.UriTest (on NYC branch)
Test: cts -m CtsNetTestCases (on NYC branch)
Change-Id: Idca79f35a886de042c94d6ab66787c2e98ac8376