Add a /product path for microg.xml search path to comply with strict build requirements

[karlbaumg]

Is your feature request related to a problem? Please describe.

In LineageOS, and other distributions relying on lineageos4microg project to include microG, we are having issues with using a /system-based path to find the microg.xml in builds that are marked as strict which doesn't let system partition to have much additions, see lineageos4microg/android_vendor_partner_gms#44

However, in microG, putting a /system/etc/microg.xml is the only way to provide build-time default settings.

Describe the solution you'd like

Another path such as /product/etc/microg.xml can be added such that microG checks that first and if it doesn't exist, then fall back to /system/etc/microg.xml so that no existing builds break and we can switch to /product/etc/microg.xml in future builds.

Describe alternatives you've considered

We tried to add it as exception but it doesn't work in strict builds lineageos4microg/android_vendor_partner_gms#46

[petefoth]

Describe the solution you'd like

Another path such as /vendor/etc/microg.xml can be added such that microG checks that first and if it doesn't exist, then fall back to /system/etc/microg.xml so that no existing builds break and we can switch to /vendor/etc/microg.xml in future builds.

At the moment, los4microg puts the file in /product/etc/, so having GmsCore look there before falling back to /system/etc would be best for us 😄

[steadfasterX]

.. and for me system_ext/etc/ would be the best place ;)
according to https://source.android.com/docs/core/architecture/partitions/product-interfaces#about-the-artifact-path-requirements system_ext or product are the correct placements for this these days so it would be great to have the choice even (if possible).

[ale5000-git]

I think, if possible, it would be best if it first search on the same partition of the apk (whatever it is) and then fallback to system.
It would be rather strange to have it in a partition different from the apk.