Ignore register existing app id for sdk library

addPackageLPw() should return a PackageSetting if the package is a sdk
library and the feature "Disallow sdk libraries to be apps" is enabled.

Bug: 328104784

Test: atest CtsSdkSandboxHostSideTests
Test: atest CtsPackageManagerTestCases
Test: atest FrameworksMockingServicesTests:com.android.server.pm
Change-Id: Ie42ecaa14488c03982c591585e5c07d3daf24ba8