WebView: remove non-inclusive terminology from docs

No change to logic, only docs. This removes non-inclusive terminology
("whitelist") from our API javadoc, with the exception of references to
setSafeBrowsingWhitelist() (which we plan to deprecate soon, in favor of
an inclusively named API).

This rephrases one paragraph to be a bit clearer as well.

Bug: 160928306
Test: m offline-sdk-docs -j4
Change-Id: If249be8a70b01c01390ebfeb418fd7c5e119f1f2