ART: Add "explicit" to constructors

Add explicit to two constructors with single argument.

Bug: 32619234
Test: m
Change-Id: I6ee6c8264d76017247e2236170c776c801719a0a
diff --git a/runtime/thread.h b/runtime/thread.h
index 75b5b12..4f26803 100644
--- a/runtime/thread.h
+++ b/runtime/thread.h
@@ -1567,7 +1567,8 @@
 
 class SCOPED_CAPABILITY ScopedAssertNoThreadSuspension {
  public:
-  ALWAYS_INLINE ScopedAssertNoThreadSuspension(const char* cause) ACQUIRE(Roles::uninterruptible_) {
+  ALWAYS_INLINE explicit ScopedAssertNoThreadSuspension(const char* cause)
+      ACQUIRE(Roles::uninterruptible_) {
     if (kIsDebugBuild) {
       self_ = Thread::Current();
       old_cause_ = self_->StartAssertNoThreadSuspension(cause);