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

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 https://github.com/lineageos4microg/android_vendor_partner_gms/issues/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 https://github.com/lineageos4microg/android_vendor_partner_gms/pull/46

Start with SettingsProvider.kt at the linked lookup for /system/etc/microg.xml and inspect how the default settings file is located. Support /product/etc/microg.xml as the preferred path while retaining the existing /system/etc/microg.xml fallback; done means both strict product-based builds and existing system-based builds can find the file.