AndroidRuntime: Expose javaAttachThread / javaDetachThread.

Add a new header library, libandroid_runtime_threads_headers,
that exposes javaAttachThread / javaDetachThread.

Also change the return value from int to bool, since the client
may not know about JNI_OK.

Test: boots
Bug: 190450693
Change-Id: Ifedd34a8c8a1847d89c9f2f7919ff7e4b13b7446