chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/.github/workflows/android.yml

@@ -0,0 +1,23 @@
+name: android
+on:
+  push:
+    branches:
+      - mobile-staging
+      - 'release/**'
+jobs:
+  test:
+    runs-on: macos-latest
+    steps:
+      - uses: actions/setup-go@v1
+        with:
+          go-version: "1.14"
+      - uses: actions/checkout@v2
+      - run: brew install --cask android-sdk
+      - run: echo y | sdkmanager --install "platforms;android-29"
+      - run: echo y | sdkmanager --install "ndk;21.3.6528147"
+      - run: ./build-android.bash
+        env:
+          ANDROID_HOME: /usr/local/Caskroom/android-sdk/4333796
+      - run: ./publish-android.bash
+        env:
+          BINTRAY_API_KEY: ${{ secrets.BINTRAY_API_KEY }}