llkd: handle 'adbd shell setsid' to preserve adbd

A zombie setsid process occurs when adb shell setsid <command> is
issued, however llkd can only detect if it is a result of a kernel
livelock by killing the associated parent, which would be adbd;
resulting in the adb connection(s) being terminated.  Will special
case this condition in order to preserve adbd for debugging purposes.

We parse <parent>&<child> in ro.llk.blacklist.parent as this
association, thus adbd&[setsid] covers this special case.
Ampersand was selected because it is never part of a process name,
however a setprop in the shell requires it to be escaped or quoted;
init rc file where this is normally specified does not have issue.

getComm() is effectively pure, so hold on to the return value for
sake of efficiency.

This also reverts commit 599958d1148c1137bee925f4cf08eda7f8050d98
which granted adbd blanket parent immunity from monitoring on
userdebug builds.  The new logic is a more refined means of
preserving the live lock checking associated with adbd and allows
the operation to be performed on user builds.

POC: date ; adb shell setsid sleep 900 ; date
Positive for bug, reports less than 15 minutes, otherwise solved.

Test: llkd_unit_test
Bug: 120983740
Change-Id: I6442463a48499d925a3a074423a24a1622905559

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