Add SafeBrowsingResponse abstract class

This adds a new class called SafeBrowsingResponse to replace the
ValueCallback parameter for WebViewClient#onSafeBrowsingHit. This
class contains specific methods for each of the available actions.

This also removes the SAFE_BROWSING_ACTION_* constants from the
WebViewClient class.

Bug: 62723291
Test: N/A
Change-Id: Ie4fe878ea470f7aea7a716ae9edb80b53b73e172