ART: Add currentThread cutout to Unstarted Runtime

Add currentThread() and getThreadState() cutouts to the unstarted
runtime to allow further compile-time initialization. The cutouts
are protected by call-stack checks.

Add tests.

Bug: 34956610
Test: m test-art-host-gtest-unstarted_runtime_test
Change-Id: I6335bccda8bedae90376fc7c47b303576f1ac78b