Replace all Android.mk prebuilt rules with Android.bp android_app_import
modules. presigned: true in Soong guarantees APK signatures are untouched
(no signapk run, alignment-safe zipalign only), which is required so
microG/FDroid retain their own certs rather than being re-signed with the
platform key.
Fix vendorsetup.sh download paths to match the reorganised directory
structure: GmsCore and GsfProxy moved under apps/GMS/, FDroidPrivilegedExtension
moved under apps/FDroid/FDroidPrivilegedExtension/. The download_apk helper
now accepts an explicit destination path instead of deriving it from the
component name.
Also stage the directory renames and removal of the unused products/ stubs.
