Always setup the native bridge library

... even if the string is empty. This will initialize the native bridge
library but mark it as unavailable.

- also, rename native_bridge_library_path to
native_bridge_library_filename to be closer to the actual meaning (it's
just the filename without any path).

Bug: 16404669.
Change-Id: I94628639691459d48d1fbf0841f36b68d51818e7