ADFA-4592: Set clean: false on build checkout for incremental :app builds

The real cause of the flat debug build time: actions/checkout defaults to
clean: true, which runs 'git clean -ffdx' at the start of every run and wipes
app/build/ (and every other module build/ dir), so :app is recompiled from
scratch each build and no incremental compilation ever happens. The rm -rf
app/build steps removed earlier were redundant with this.

Set clean: false on the build_apk checkout so build/ dirs persist across runs
on the self-hosted runner, enabling Gradle incremental compilation. Safe now
that the custom registerD8Task (which wrote untracked dex and caused the
original stale-artifact D8 crashes) has been removed in this PR.

Author: hal-eisen-adfa

.github/workflows/debug.yml

@@ -91,6 +91,11 @@ jobs:
         with:
           ref: ${{ github.event.pull_request.head.ref }}
           fetch-depth: 0
+          # Preserve build/ dirs between runs on the persistent self-hosted
+          # runner so Gradle compiles :app incrementally instead of from
+          # scratch. Default (clean: true) runs `git clean -ffdx`, wiping every
+          # module's build/ dir on every run and forcing a full :app rebuild.
+          clean: false
 
       - name: Validate branch name length
         run: |