Revise android apk target handling in mb again.
Revise android apk target handling in mb again.

The isolate recipe module looks for <target>.isolated.gen.json, e.g.
https://build.chromium.org/p/chromium.android/builders/Android%20Swarm%20Builder/builds/1062,
and there's no clean way I can see to have it look elsewhere based on
whether gyp or gn is configured in mb for a given bot.

BUG=

Review URL: https://codereview.chromium.org/1659963002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#373305}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk