ART: Properly check for attached thread in CheckJNI

The previous implementation checked too late, as the required
ScopedObjectAccess would have already aborted. Instead, preface
all necessary functions with an explicit check.

Performance should be almost neutral, as the test is only moved
earlier, but not duplicated.

Bug: 78135453
Test: m test-art-host-gtest-jni_internal_test
Change-Id: I7b98744e9a3895e84ba9e2661975ce29722076c3