logd: move current sequence from LogBufferElement to LogBuffer

This is required for tests that are aware of sequence numbers to pass;
each new LogBuffer instance should start from sequence = 1, which
isn't the case if the current sequence number is a static.

Test: unit tests
Change-Id: Ie488f8ac5e22b946b7e6237d1d5caf14929c0ec3
diff --git a/logd/LogReader.cpp b/logd/LogReader.cpp
index 89562a4..234ddc7 100644
--- a/logd/LogReader.cpp
+++ b/logd/LogReader.cpp
@@ -200,7 +200,7 @@
             if (nonBlock) {
                 return false;
             }
-            sequence = LogBufferElement::getCurrentSequence();
+            sequence = log_buffer_->sequence();
         }
     }