AAPT: Move private attrs to new type for framework

Private attributes are typically placed after public
attributes in the resource table. Each time a new version
of the Android framework is released, new public attributes
take the place of the private attributes, and the private
attributes are shifted after the new public ones.

This means that any apps built against the newer SDK
may inadvertently be using private attributes on older
devices.

This change moves all private attributes to a completely
different type ID, so there will never be collisions across
versions.

These private attributes are automatically moved to a synthesized
type only for the system resources.

Bug:18263655

Change-Id: I7a850512953fadcc9f3524d509cea30249782db8