Fix scanning APKs from APEX installed in /data.

The preinstalledModulePath field was added exactly for this purpose, but
I mistakenly used the modulePath field, which in turn was causing
PackageManager to not scan APEXs activated from /data since /data is not
one of the eligible system partitions.

Bug: 138429615
Test: Manual. Self reminder to add this to CTS tests.
Change-Id: If3cf5b666c3ed8215331383e08d958ae31e6206e