Fix the isSystemApp flag for APEX

To be consistent with the behavior of system APKs. An updated
APEX (in /data) is still considered as a system app.

Fix tests by checking the FLAG_UPDATED_SYSTEM_APP flag instead
against updated APEX.

Bug: 225760014
Bug: 225756739
Test: atest CtsStagedInstallHostTestCases \
            com.android.server.pm.ApexManagerTest \
	    GtsStagedInstallHostTestCases
Change-Id: Idc4d9976cb0e075f9ddcd59378314c9941fd9fa5