Allow splits to declare a classloader type.

Add an attribute android:classLoader which can be set to the name
of the classloader the base or split wishes to use. For now, this can
only be set to "dalvik.system.PathClassLoader" or
"dalvik.system.DelegateLastClassLoader". The current implementation
only allows the classloader to exist in the boot classpath but that
restriction can be loosened in a future change if necessary.

Test: cts-tradefed run cts-dev -m CtsAppSecurityHostTestCases
Bug: 36044779

Change-Id: I5a51f7aa0c8c4c398d2f46129eb06785cc8e3c3f