logd: refine is_prio

- Heuristics associated with translation of kernel messages to
  Android user space logs.
- Limit is_prio to 4 characters, we got false positives on hex
  values like <register contents> with no alpha chars.
- x11 and other register definitions are not valid tags, en0 is
- fix some Android coding standard issues

Change-Id: Idc3dcc53a2cb75ac38628c8ef7a5d5b53f12587a