add unittests for libnativeloader

libnativeloader hasn't had a unit test. Now we have it. The unittest
mocks calls to libdl, libbridgedloader, and JNI using gmock.

Not all tests are added yet. To ease the review, a few tests for core
scenarios are added.

Bug: 130388701
Test: libnativeloader_test

Change-Id: I518512047da66b44bc99cec26e2c34ba4985cca3