Fix getPackageInfo("android") in the system server.

The fix for bug 16555230 attempted to return the same LoadedApk
object between the system context object and the activity thread's
list of opened packaged. This ensured that we used the same classLoader
on both these objects (among other things).

However, the object the Context owns is actually incomplete (and
necessarily so) because it might be created before the package
manager is and we need the package manager to construct a complete
LoadedApk object.

Work around this by manually setting the classloader and aInfo for
"android" contexts created by the system server's activity thread.

bug: 18167468
Change-Id: I623634f912111f89eaf716b8258d1926d2513dcd