[native bridge] Make sure we always unload the native bridge

libnativebridge may allocate some resources during loading and
intialization and de-allocate them when unloading. This makes sure that
we don't leak anything.

Bug: 18097480

(cherry picked from commit 07d83c7a)

Change-Id: I3a5155a6760399ccfaf9130c72679615a3d4f2e7