Add a sample app that uses Compose.

[zach-klippenstein]

Based on #50.

Note that Compose is still not supported, but having this sample in place
ensures that we keep the build working for Compose going forward.

Example of output for an app that has a Compose view with a nested Android view:

com.squareup.radiography.sample.compose.MainActivity:
window-focus:false
 DecorView { 1080×2160px }
 +-LinearLayout { 1080×2028px }
 | +-ViewStub { id:action_mode_bar_stub, GONE, 0×0px }
 | `-FrameLayout { 1080×1962px }
 |   `-FitWindowsLinearLayout { id:action_bar_root,
 |     +-ViewStubCompat { id:action_mode_bar_stub, GONE,
 |     `-ContentFrameLayout { id:content, 1080×1962px }
 |       `-AndroidComposeView { 770×425px, focused }
 |         `-AndroidViewsHandler { 770×425px }
 |           `-ViewBlockHolder { 160×53px }
 |             `-TextView { 160×53px, text-length:9 }
 +-View { id:navigationBarBackground, 1080×132px }
 `-View { id:statusBarBackground, 1080×66px }

This will get better in #33!

This PR is 3 commits:

1. Run UI tests on all PRs, not just ones targeting main.
2. Add the sample app.
3. Add smoke UI tests to both the new sample app and the existing one to ensure they launch.

[zach-klippenstein]

The red shard actually passed, it just failed to upload the success report.

[rjrjr]

Reviewing these in the wrong order! Oh well.

I already miss the 🥖