Support adding System Traces to Wear Bugreports

System Traces collected with the native System Tracing App (Traceur) are
now added under a separate directory (systraces) for Wear bugreports.
Note that this change is functionally no-op for non-Wear form-factors (changes only one log's wordings for non-Wear form-factors).

Bug: 183239853
Test: manual
Change-Id: Id6b9aa0d38a0d465b763ec86fb1192875379431c
(cherry picked from commit 7111510c691ccf20baf372dab5cea2ac30ae0c23)