Previously when an installer package was uninstalled we removed all
reference to it in InstallSource.
Now we retain the initiator package name (but not installer or
originator). We set a flag to remember it has been uninstalled, and we
also store its signatures which would otherwise cease to be
available. Update persistence logic.
Also minor refactoring of InstallSource#create to separate the normal
case from those that need to set every field.
This information is not yet exposed in the API, but it will be.
Test: atest PackageManagerTests PackageVerifierTest
Test: Manually verify XML contents before & after uninstalling an installer.
Bug: 134746019
Bug: 146555198
Change-Id: I1452402de554b916e5f79bd386c5857d4860ba12
72b7e673c7