Add integration test support for @utf8InCpp

This patch adds validation of String and String[] marked with
@utf8InCpp.

While here, move everything but main() in aidl_test_service.cpp to an
anonymous namespace.

Bug: 26729450
Test: Integration, unittests continue to pass

Change-Id: Iab79bc23ed8a34168544425952afd2e29cdbe030
8 files changed