Making this library double loadable allows it to be linked to from
multiple Framework libraries linked into a single client APK. Without
this change, configurable flags can only be used inside Framework
services because only this library can only be linked to from one
Framework library.

Bug: 234833109
Test: manual test with 'adb shell device_config' commands
Change-Id: I3fd48db45666d02f642534e9140ec9d6a1ce4f68