Revert "libutils: disable implicit ref base construction"

This reverts commit 9a1bf53c4b0a45ac0072d26807b145ddcf044c28.

Reason for revert: b/186028624

Change-Id: Icb8b1623df2794222ce3fe34ed5f43506ccf2f68
diff --git a/libutils/Threads.cpp b/libutils/Threads.cpp
index 1c8f810..540dcf4 100644
--- a/libutils/Threads.cpp
+++ b/libutils/Threads.cpp
@@ -693,7 +693,7 @@
     mThread = thread_id_t(-1);
 
     // hold a strong reference on ourself
-    mHoldSelf = sp<Thread>::fromExisting(this);
+    mHoldSelf = this;
 
     mRunning = true;