fix: Override vndk-ext lib name with original lib

vndk-ext should have the same output filename with the lib which it
extends. "extends" property is "module name", not "filename" of output.

Bug: 143130384
Test: add "protox" as a vndk-ext module (extends 'libprotobuf-cpp-full')
      m protox.vendor && check output filename under /vendor/lib/vndk/
      (should have correct suffix)

Change-Id: I5741ea87b4f2ad375b69f54c93dcb8753d9952dd