liblog: android_log_event_list class permit -EBUSY retry

write() method was one-shot, a second shot on -EBUSY would always
return -EBUSY even if successful.  Reset internal error if -EBUSY
when retransmitting.  write() now reports a positive count for
transmission success.  Composition errors trump transmission errors.

Test: gTest logcat-unit-tests --gtest_filter=*.descriptive while
      under heavy DOS stress levels of logging.
Bug: 31456426
Change-Id: Ib1920c3f10cf1df8ad8eb6a884724794b577b29d