Add support for missing Custom Tab implementation

Apropos #4, PR #7 fixed the case where the user has an older version of Chrome, but didn't fix the case where there's no Custom Tab implementation at all (e.g. no Chrome installed).

For example, if you create an Android Emulator on API 28 using "Google ARM64-V8a Play ARM 64 v8a System Image", you'll get a phone with no Chrome and only a minimal "WebView Browser Tester" app that doesn't support custom tabs. (The system WebView is on Chromium 66, from 2018!)

The library doesn't currently work properly in that case, even with the fallback intent filters.

To repro: On an API 28 emulator without Chrome, install the Showcase app, view the Web Authentication Session playground, and Sign In. It will open the WebView Browser Tester.

Actual: If you sign in, it will take you back to the home screen of the app.
Expected: When you sign in, it should display the result URL on the Web Authentication Session playground

Reproduce this in the Showcase app on an API 28 emulator created with the Google ARM64-V8a Play ARM 64 v8a System Image, then open the Web Authentication Session playground and sign in. Investigate the missing Custom Tab path and verify that, without Chrome, the sign-in flow displays the result URL on the playground instead of returning to the app's home screen.