Pass initiatingPackageName to verifier.

The initiating package is the actual package requesting the install,
which is what a package verifier wants. Previously we've passed the
installing package, but this can be overridden by the initiating
package.

Also mark the things needed to test this as @TestApi.

Test: atest PackageVerifierTest (as amended)
Bug: 134746019
Change-Id: I8108b8e21c90ae237afa976ff416ffae97ec08e4