Clean up use of HAVE_ANDROID_OS

HAVE_ANDROID_OS was defined as "1" for targets, but never defined as "0"
for non-targets. Changing them to #ifdef should be safe and matches
all the other uses of HAVE_ANDROID_OS throughout the system.

Change-Id: I82257325a8ae5e4e4371ddfc4dbf51cea8ea0abb
diff --git a/libs/utils/Threads.cpp b/libs/utils/Threads.cpp
index b1bd828..35dcbcb 100644
--- a/libs/utils/Threads.cpp
+++ b/libs/utils/Threads.cpp
@@ -739,7 +739,7 @@
     wp<Thread> weak(strong);
     self->mHoldSelf.clear();
 
-#if HAVE_ANDROID_OS
+#ifdef HAVE_ANDROID_OS
     // this is very useful for debugging with gdb
     self->mTid = gettid();
 #endif