liblog: add log/log_system.h

Move SLOG logging macros to their own home.

SLOGV
SLOGV_IF
SLOGD
SLOGD_IF
SLOGI
SLOGI_IF
SLOGW
SLOGW_IF
SLOGE
SLOGE_IF

Test: gTest liblog-unit-tests --gtest_filter=liblog.SLOG
Bug: 34250038
Change-Id: I456c7ad23152dc1c7b39143d87401a8737e24d64
diff --git a/liblog/tests/Android.mk b/liblog/tests/Android.mk
index 1a685df..7bbf86d 100644
--- a/liblog/tests/Android.mk
+++ b/liblog/tests/Android.mk
@@ -57,7 +57,8 @@
 test_src_files := \
     liblog_test.cpp \
     log_id_test.cpp \
-    log_radio_test.cpp
+    log_radio_test.cpp \
+    log_system_test.cpp
 
 # to prevent breaking the build if bionic not relatively visible to us
 ifneq ($(wildcard $(LOCAL_PATH)/../../../../bionic/libc/bionic/libc_logging.cpp),)