logd: validate and fill in socket credentials

- android::pidToUid() additional checking.  Make sure if we have to
  convert a PID to an UID that the parse of /proc/<pid>/status
  requires a trailing space after the number
- android::tidToPid() added, in the same vein as android::pidToUid().
- stats.tidToPid() added
- If no credentials, set PID to 0 and UID to DEFAULT_OVERFLOWUID
- If credentialed PID is 0, use stats.tidToPid()
- If credentialed UID is DEFAULT_OVERFLOWUID, use stats.pidToUid()

Test: remove +passcred from logd.rc for daemon and confirm very few
      UID=65534 or PID=0 cases actually show up
Bug: 37985222
Change-Id: I7d20506e70e67beb3043d1537cf9450ab58dc278
diff --git a/logd/LogBufferInterface.h b/logd/LogBufferInterface.h
index 7d82b91..ff73a22 100644
--- a/logd/LogBufferInterface.h
+++ b/logd/LogBufferInterface.h
@@ -33,6 +33,9 @@
     virtual int log(log_id_t log_id, log_time realtime, uid_t uid, pid_t pid,
                     pid_t tid, const char* msg, unsigned short len) = 0;
 
+    virtual uid_t pidToUid(pid_t pid);
+    virtual pid_t tidToPid(pid_t tid);
+
    private:
     DISALLOW_COPY_AND_ASSIGN(LogBufferInterface);
 };