logd: clear return and deal with busy if readers locked

- Propagate to caller the clearing errors, busy blocked by reader.
- For clear, perform retries within logd with a one second lul each,
  telling readers to skip, but on final retry to kill all readers if
  problem still persists due to block reader (or high volume logspammer).

Bug: 23711431
Change-Id: Ie4c46bc9480a7f49b96a81fae25a95c603270c33
diff --git a/logd/LogBuffer.h b/logd/LogBuffer.h
index de76693..d8e0b90 100644
--- a/logd/LogBuffer.h
+++ b/logd/LogBuffer.h
@@ -63,7 +63,7 @@
                      int (*filter)(const LogBufferElement *element, void *arg) = NULL,
                      void *arg = NULL);
 
-    void clear(log_id_t id, uid_t uid = AID_ROOT);
+    bool clear(log_id_t id, uid_t uid = AID_ROOT);
     unsigned long getSize(log_id_t id);
     int setSize(log_id_t id, unsigned long size);
     unsigned long getSizeUsed(log_id_t id);
@@ -86,7 +86,7 @@
 
 private:
     void maybePrune(log_id_t id);
-    void prune(log_id_t id, unsigned long pruneRows, uid_t uid = AID_ROOT);
+    bool prune(log_id_t id, unsigned long pruneRows, uid_t uid = AID_ROOT);
     LogBufferElementCollection::iterator erase(
         LogBufferElementCollection::iterator it, bool engageStats = true);
 };