system/core: preparation to pull back interfaces from android/log.h

Point to log/log.h where necessary, define LOG_TAG where necessary.
Accept that private/android_logger.h is suitable replacement for
log/logger.h and android/log.h.

Correct liblog/README

Effectively a cleanup and controlled select revert of
'system/core: drop or replace log/logger.h' and
'system/core: Replace log/log.h with android/log.h'.

Test: compile
Bug: 30465923
Change-Id: Ic2ad157bad6f5efe2c6af293a73bb753300b17a2
diff --git a/debuggerd/signal_sender.cpp b/debuggerd/signal_sender.cpp
index 3adbef2..7fe4dee 100644
--- a/debuggerd/signal_sender.cpp
+++ b/debuggerd/signal_sender.cpp
@@ -14,7 +14,10 @@
  * limitations under the License.
  */
 
+#define LOG_TAG "debuggerd-signal"
+
 #include <errno.h>
+#include <pthread.h>
 #include <signal.h>
 #include <stdlib.h>
 #include <string.h>