[CI] (.github/workflows): run 16 emulator retrace jobs at a time

- 231 jobs at 4 a time took 182 of the run's 198 minutes while 16 runners sat idle

.github/workflows/apk.yml

@@ -410,7 +410,9 @@ jobs:
       ANDROID_SDK_ROOT: ${{ github.workspace }}/.android/sdk
     strategy:
       fail-fast: false
-      max-parallel: 4
+      # The account runs at most 20 jobs at once. 16 lets the ~230 emulator jobs use most of the
+      # pool once Test is done, and leaves Test some room while both workflows run.
+      max-parallel: 16
       matrix: ${{ fromJSON(needs.trace-cases.outputs.android) }}
     steps:
       - name: Set swap space