liblog: Instrument logging of logd write drops

- If logger system is prostrated, send an event message with the
  liblog tag from the associated UID and PID with a count of
  dropped messages once logging is resumed.
- Added to the README a description of the error return values.
- Describe in the README the appropriate mitigations for dropped
  messages.
- If the caller sees this message, then
  /proc/sys/net/unix/max_dgram_qlen is likely too small

Change-Id: Iaf387b9e5e1b6aa93bebc7481f9e8353732e3229