Prevent non-admin users from deleting system apps.

This addresses a security issue where the guest user can remove updates
for system apps.

With this CL, attempts to uninstall/downgrade system apps will fail if
attempted by a non-admin user, unless the DELETE_SYSTEM_APP flag is
specified.

This is a fixed version of ag/17352264, to address b/236578018.

Bug: 170646036
Test: atest DeletePackageHelperTest
Merged-In: I55a3d322e84aaaced69bd5edeaf90e82ac9a0dd3
Change-Id: I0fd9bfaebccc583f641db4416c9021d47f6fcf44