Load libart.so with RTLD_NODELETE

During shutdown of libart.so it is possible that threads do not return
out of libart.so code prior to JNI_DestroyJavaVM returning. In order
to prevent segfaults never unmap libart.so.

Bug: 28406866

Change-Id: If52898a5a94f2e41c6634fb4e2cc7059e6436ca4