Issue 5697: getContextClassLoader returns a non-application classloader

ActivityThread should try to set the value for Java
Thread.getContextClassLoader to the PathClassLoader that loaded the
APK's classes so that Java frameworks that use the Java context class
loader, which is not to be confused with the
android.content.Context.getClassLoader which serves a similar purpose
in the Android framework.

However, we avoid setting the Java context ClassLoader to the APK's
PathClassLoader if there is a static indication that multiple packages
may share the VM, since they could load in an unpredictable order
leading to different values for the thread local Java context
ClassLoader. In this case, we instead use a specially created
WarningContextClassLoader that warns the user the first time the Java
context ClassLoader.

Currently the static indications that a package might share a VM with
other packages are the presence in the AndroidManifest of a
sharedUserId or requesting a non-default application process name.