logd: fix bug in FlushTo when requesting exact sequence number

SimpleLogBuffer::FlushTo() attempts to find the iterator matching a
given sequence number, but the logic is wrong and will always skip one
element forward.  This change fixes this and adds a test for the
situation.

This likely contributed to some test instability in the past, but was
identified because subsequent changes that track the start value
closer exacerbated this issue.

Test: existing and new unit tests
Change-Id: Iba2e654e94234693dba20d4747a60bc79d195673
diff --git a/logd/SimpleLogBuffer.cpp b/logd/SimpleLogBuffer.cpp
index 8a11b92..ceecc6d 100644
--- a/logd/SimpleLogBuffer.cpp
+++ b/logd/SimpleLogBuffer.cpp
@@ -126,7 +126,9 @@
         for (it = logs_.end(); it != logs_.begin();
              /* do nothing */) {
             --it;
-            if (it->getSequence() <= start) {
+            if (it->getSequence() == start) {
+                break;
+            } else if (it->getSequence() < start) {
                 it++;
                 break;
             }