[ECM] Rename getPkgName to getPackageName

In SignedPackage and SignedPackageParcel, rename the getPkgName() method
and pkgName field to getPackageName() and packageName.

This is an API change. Fortunately, there is only one consumer of
this API: EnhancedConfirmationService. (Both currently and planned
for future.)

Fix: 325048954
Test: atest FrameworksServicesTests:com.android.server.systemconfig.SystemConfigTest
Change-Id: I3961cadf7e42ce1641f3c89ddaf818225f319390