Make WebView data directory configurable.

Add WebView.setDataDirectory(String suffix) and WebView.disableWebView()
interfaces. These will enable applications to better control the use of
WebView when running multiple processes, since it's not safe for more
than one process in an app to use the same data directory.

disableWebView is completely implemented in this change;
setDataDirectory will require WebView-side implementation changes to be
functional.

Bug: 63748219
Test: cts android.webkit.cts.WebViewDataDirTest
Change-Id: Ieff225dc99b6b3ca5f223c35f7a854f0c53692df