logcat: allow comma-separate list of buffers

(cherry pick from commit 45177732)

- Add parsing to support comma+ separated list of buffers.
- Move get_size test into a standalone get_groups function
- add some additional tests for this feature, leveraging
  get_groups to confirm buffer selection.

Bug: 28120456
Bug: 28788401
Bug: 28813587
Change-Id: I0b42736c08cf4b2a435cb74cda540dc163a26bd1