Add developer settings link to WebView DevTools

Adds a convenience button in Android's developer settings to launch
WebView's DevTools UI. This avoids the need to use ADB shell or to
follow links from another app (such as WebView Shell or an unstable
WebView version) to access the stable WebView DevTools.

Bug: 371520781
Test: atest SettingsRoboTests:com.android.settings.development.WebViewDevUiPreferenceControllerTest
Flag: EXEMPT new entry point to a feature which already exists
Co-Authored-By: Adam Walls <avvall@google.com>
Change-Id: I0c3d628b2833aa0b59aae091215e8aa660f5bfe8