logd: security buffer only AID_SYSTEM reader

- limit AID_SYSTEM uid or gid to read security buffer messages
- adjust liblog tests to reflect the reality of this adjustment

To fully test all security buffer paths and modes

$ su 0,0,0 /data/nativetest/liblog-unit-tests/liblog-unit-tests --gtest_filter=liblog.__security*
$ su 1000,1000,1000 /data/nativetest/liblog-unit-tests/liblog-unit-tests --gtest_filter=liblog.__security*
$ su 2000,2000,2000 /data/nativetest/liblog-unit-tests/liblog-unit-tests --gtest_filter=liblog.__security*

ToDo: Integrate the above individually into the gTest Q/A testing

Bug: 26029733
Change-Id: Idcf5492db78fa6934ef6fb43f3ef861052675651
diff --git a/logd/LogBuffer.cpp b/logd/LogBuffer.cpp
index 9e0d451..8c30f79 100644
--- a/logd/LogBuffer.cpp
+++ b/logd/LogBuffer.cpp
@@ -907,7 +907,8 @@
 }
 
 uint64_t LogBuffer::flushTo(
-        SocketClient *reader, const uint64_t start, bool privileged,
+        SocketClient *reader, const uint64_t start,
+        bool privileged, bool security,
         int (*filter)(const LogBufferElement *element, void *arg), void *arg) {
     LogBufferElementCollection::iterator it;
     uint64_t max = start;
@@ -938,6 +939,10 @@
             continue;
         }
 
+        if (!security && (element->getLogId() == LOG_ID_SECURITY)) {
+            continue;
+        }
+
         if (element->getSequence() <= start) {
             continue;
         }