Fix symlinks to system libs for flattened apex

Symlinks to system libs should be created for flattened apex regardless
that it is primary or not.

For example, GSI installs non-primary flattened apexes as well. These
flattened (non-primary) apexes could be activated on non-updatable
devices.

Bug: 148195518
Test: GSI runs on P
Change-Id: I238b226473d923e03280b1b28dd0d5d1f77ae74a