android: look for debug/vendor prefixed options
Properties from the vendor partition must use a "vendor." prefix from Android T+, see https://source.android.com/docs/core/architecture/configuration/add-system-properties#add-vendor-properties.
Meanwhile the "debug." prefix can be used for local overrides.
The order of precedence thus becomes:
- getenv
- debug.mesa.*
- vendor.mesa.*
- mesa.* (as a fallback for older versions)