Refactor unit test for Android Runtime (Boot) device configuration flags.

This is in preparation of checking other device configuration flags.

Test: core/jni/runtime_native_boot-flags-test.sh
Bug: 72446017
Bug: 120794191
Bug: 123754583
Bug: 119800099
Change-Id: I14d6defe5f7e9fcf2f3dd27a4d0fae81f2aec596