[ART] Move to system/core native bridge library

Some cleanups on the way:
- move NativeBridgeRuntimeCallbacks from a global to a field
- rename native_bridge_library_string to the more suggestive
navtive_bridge_library_path
- clean up imports

Bug: 16884833
Change-Id: I73aab8e212860ba5aee9444d801806d3da326a41