WebView: add WebView#getSafeBrowsingPrivacyPolicyUrl() API

This adds a dedicated API for fetching a URL for the Safe Browsing
reporting privacy policy.

This URL should be displayed by applications on custom Safe Browsing
interstitials.

Bug: 64077668
Test: make update-api (it compiles and javadocs look good)
Change-Id: I2873260edcaa924e68517c8679079e147b9995f4