Remove exception from OverlayConfig zygote method

The AssetManager constructor is marked UnsupportedAppUsage and the
recent changes to OverlayConfig caused an exception to be thrown when
the function is not called from the root process.

Previously a warning message would be logged indicating that no static
overlays would be loaded. Removing the uid check will restore the
previous behavior.

Also remove the system uid checks for initializeSystemInstance so PMS
service can also be constructed during testing.

Bug: 149806146
Test: invoke new AssetManager() from non-root thread
Change-Id: I55af2cddeba4df7a99c281c93e0181b9e2139139