logpersist: switch to pgrep from ps

Changes expected output from a line cut from the less than portable
ps output to reporting the pid of logcatd (uid=logd comm=logcat).
Preserve reporting an error code if logcatd did not start.

Bug: 29075536
Change-Id: I61aa0a4787d4594c415d2c7db5bbb0e512676bf8