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/LogBuffer.h b/logd/LogBuffer.h
index 6274051..7f1e128 100644
--- a/logd/LogBuffer.h
+++ b/logd/LogBuffer.h
@@ -53,4 +53,6 @@
     virtual bool Clear(log_id_t id, uid_t uid) = 0;
     virtual unsigned long GetSize(log_id_t id) = 0;
     virtual int SetSize(log_id_t id, unsigned long size) = 0;
+
+    virtual uint64_t sequence() const = 0;
 };
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