Clean up the test mocks.

Split them up so that the API tests only have the JNI mock that they
need, and the "platform" mock - which is nonfunctional there - is not
present.

Also a few other code cleanups.

Test: atest libnativeloader_test \
            art_libnativeloader_cts_test \
            libnativeloader_lazy_test
Bug: 335224367
Change-Id: Ifabc64db43e0346d5aefc9809266190690178ebf
5 files changed