libnativehelper: refactoring for Runtime Module

This change puts a C ABI between the caller and libnativehelper which
means each could link separate libc++ library instances using
namespaces.

Bug: b/119840313
Test: Builds & boots
Test: atest JniInvocation_test
Test: atest libnativehelper_api_test

Change-Id: Idaa4ddd3118cbb4f3997abdcf355220ef80eb775