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

These flags live in device configuration namespace
`runtime_native_boot`. Exercise them by checking that they are passed
to the zygote process(es) as a runtime option after a reboot.

At the moment this test only checks flag `gctype`, which is the only
flag currently implemented in namespace `runtime_native_boot`.

Test: core/jni/runtime_native_boot-flags-test.sh
Bug: 72446017
Bug: 120794191
Bug: 123524494
Bug: 123754583
Change-Id: I6cf43c3ee0d405fb1b69a450587dd712f911358d