Revert "Merge pull request #16325 from guilherme-pg/findmydevice"

This reverts commit 2bbaa0d5a75c65dc30744c91b82f8cafc3406c53, reversing
changes made to 5e89c55e77eb7b5d29f7cca13a114e3b74ecdd30.
Revert "Bug 897352 - [gaia build system] Transition to a build_stage for assembling apps"

This reverts commit d17a39150f09378ed145be605857ff2254da516e.