logd: reader/writer element locks

Switch to a reader writer lock for the Element List lock.  Also setup
for a reader writer lock for the Times list, but continue to use a
mutex where rdlock() and wrlock() are the same implementation for now.

This should improve general reader performance and prevent blocking of
other reader operations or exit by a single hung logd.reader.per
thread.  For example, a full length logcat of an empty buffer (eg:
crash log buffer) will hold a lock while the iterator scans the entire
list.

Test: gTest liblog-unit-tests, logd-unit-tests, logcat-unit-tests
Bug: 37378309
Bug: 37483775
Change-Id: If5723ff4a978e17d828a75321e8f0ba91d4a09e0
diff --git a/logd/LogStatistics.cpp b/logd/LogStatistics.cpp
index 3f020b6..0b6b28c 100644
--- a/logd/LogStatistics.cpp
+++ b/logd/LogStatistics.cpp
@@ -228,6 +228,7 @@
 }
 
 // caller must own and free character string
+// Requires parent LogBuffer::wrlock() to be held
 const char* LogStatistics::uidToName(uid_t uid) const {
     // Local hard coded favourites
     if (uid == AID_LOGD) {