Move BACKTRACE_NO_TID to BACKTRACE_CURRENT_THREAD.

Use a bit better name for this. The other name was a bit confusing.

Change-Id: I1261f2ee3854a9c8b82133ad0bfbbbe48b43c9ac

(cherry picked from commit 242b1a8c7a5ec73172bac811e51acbc27201f22c)

Conflicts:
	libbacktrace/Backtrace.cpp
diff --git a/include/backtrace/backtrace.h b/include/backtrace/backtrace.h
index 186d327..fa81f21 100644
--- a/include/backtrace/backtrace.h
+++ b/include/backtrace/backtrace.h
@@ -28,8 +28,8 @@
 // thread from the current process will be traced.
 #define BACKTRACE_CURRENT_PROCESS -1
 // When the tid to be traced is set to this value, then trace the specified
-// pid.
-#define BACKTRACE_NO_TID -1
+// current thread of the specified pid.
+#define BACKTRACE_CURRENT_THREAD -1
 
 #define MAX_BACKTRACE_FRAMES 64