[WebView] Add documentation regarding geolocation api on secure origins.

Update to javadoc comments saying that for apps targeting N and up
the geolocation api is only supported on secure origins.
On non-secure origins requests to geolocation api are automatically
denied without calling the corresponding
WebChromeClient#onGeolocationPermissionsShowPrompt method.

BUG: 28080869
Change-Id: I57726f7f3b3d38db0535f742b1f74c40c1f1ce8b