nativeloader: Add Reset capabilities

In the case when VM is restarted all weak references
from the old VM become invalid. In such event NativeLoader
needs to clear the list of classloaders from old VM.

Bug: http://b/28453840
Bug: http://b/28449304
Change-Id: I2268c1e21cf940c57ddc5f0312f56b71aa702134
(cherry picked from commit be4ca3af)