Add initial unit tests for WebViewUpdateService.

The logic in the WebViewUpdateService is now more complex and there
are lots of edge cases that should be tested to make sure we don't
regress anything.

Also refrain from running fallback logic on boot if fallback logic not
enabled (bug found through a failing test, yeeah buddy!).

Catch uncaught MissingWebViewException (very timing-dependent!) and add
test for this.

Add tests for:
Some package setups (single package, non-default vs. default packages,
    valid vs. invalid packages).
Ensure correct error codes used
Switching provider through settings-method
Switching provider through adding more prioritized provider
Switching provider in the middle of already running webview preparation
Ensure fallback logic is run when enabled and not run when disabled (at
    boot, packageStateChanged, and when adding a new user).

Bug: 27635535

Change-Id: I275ecb0f6129f71258da0fa053e7f7b36a675886