WebViewUpdateManager: handle devices without WebView properly.

Devices without FEATURE_WEBVIEW (Wear) don't run the
WebViewUpdateService and so looking up the service binder via
SystemServiceRegistry fails. This returns null in most cases but for
system processes this triggers a WTF, and was causing the Wear settings
app to crash.

Instead of trying to handle WebViewUpdateManager being null, guard all
the usages with an explicit check for whether WebView is supported on
the device, and update the API docs to clarify this requirement.

Bug: 319292658
Bug: 360768638
Test: atest CtsWebkitTestCases
Flag: android.webkit.update_service_ipc_wrapper
Change-Id: I13f0ddde49b3866ceb747a1d1b11e45504f05640