Refactor Zygote runtime option checking in runtime_native_boot-flags-test.sh.

Generalize runtime option checking functions, so that they can be used
to check options other than the ones starting with `-Xgc:`

Test: core/jni/runtime_native_boot-flags-test.sh
Bug: 72446017
Bug: 120794191
Bug: 123754583
Bug: 119800099
Change-Id: I263a177486f8e24ce92928973f7c8853baf0983b