Change WebView fallback mechanism to consider all users' package states.

MultiProcess WebView will use a Service to start a separate process (the
renderer process). This Service can only be started if the WebView
package is enabled for the current user. To ensure that the current
WebView package is usable by all users on a single device we now only
use a WebView package as WebView implementation if that package is
enabled and installed for all user on the device. This also means that
the WebView-fallback mechanism will trigger when disabling the primary
WebView package for any user (not just the system user).

Also add multi-user unit tests to cover this new change.

Bug: 32894152
Test: run unit tests in WebViewUpdateServiceTest
Test: ensure the standalone WebView package becomes enabled (for all
device users) when disabling Chrome for a secondary user.
Test: load WebView (both using Monochrome, and using the standalone
WebView).

Change-Id: Iad3fc48aa50273062c2f29ae48a343c2dea38116