WebView: rename initSafeBrowsing and remove shutdownSafeBrowsing

This renames the method as follows:

 * initSafeBrowsing -> startSafeBrowsing

This also updates documentation to fix javadoc references, and to
clarify that Safe Browsing checks are not guaranteed until after
startSafeBrowsing() invokes its callback.

This does not change the method name in WebViewFactoryProvider, because
changing this would break CTS tests (since the WebView APK implements
this under the old name).

This also removes shutdownSafeBrowsing, because we found there was no
good use for this (it's meaningless for AOSP, and we were recommended to
avoid calling the underlying GMSCore API for GoogleWebView). More
significantly, there's no good place for an application to call it from.

Bug: 64331900
Test: make update-api (docs are correct)
Change-Id: Idf2b9390306052f5c5dfb92909fca4cfeec74aef