logcat: expand -n, -r and -b

- Add property expansion to the -n/--rotate-count,
  -r/--rotate-kbytes and -b/--buffer parameters.
  e.g. -r '${logcat.rotate-count:-6}' will expand the property
  logcat.rotate-count, and if not present will default to 6
- Add gtest to confirm mid-stream expansion
- No longer support ",:;<whitespace>" for buffer tokenization,
  settling on only supporting ",".

Bug: 28120456
Change-Id: I7ebb6146f72047631536c457952fa50d0abb98ab