More adb buffer fixes.

This patch factors out a lot of the basic protocol code: sending OKAY,
sending FAIL, and sending a length-prefixed string.

ADB_TRACE has been non-optional for a long time, so let's just remove
the #ifs.

Also actually build the device tracker test tool (and remove its duplicate).

Bug: http://b/20666660
Change-Id: I6c7d59f18707bdc62ca69dea45547617f9f31fc6
diff --git a/adb/commandline.cpp b/adb/commandline.cpp
index 4941689..7964cd3 100644
--- a/adb/commandline.cpp
+++ b/adb/commandline.cpp
@@ -490,17 +490,8 @@
         printf("\n");
     }
 
-    // TODO: should this be adb_status?
-    char buf[5];
-    if(!ReadFdExactly(fd, buf, 4)){
-        fprintf(stderr,"* error reading response *\n");
-        adb_close(fd);
-        return -1;
-    }
-    if(memcmp(buf, "OKAY", 4)) {
-        buf[4] = 0;
-        fprintf(stderr,"* error response '%s' *\n", buf);
-        adb_close(fd);
+    if (!adb_status(fd, &error)) {
+        fprintf(stderr,"* error response '%s' *\n", error.c_str());
         return -1;
     }