Don't kill children of adbd early in the shutdown sequence

Children of adbd (things executed on the shell or directly via adb
commands like adb shell or adb root) get different process group id of
their own. So, by definition, they are untracked processes.

However, we shouldn't kill them early in the boot process, especially
before adbd is gone.

Otherwise, the host-side component (people or any automated script)
would noitce the death of the command, before the disconnection event.
Then they can't tell if the command was actually failed or successfully
ran and the connection is lost.

Bug: 433451565
Bug: 434101714
Test: run v2/wear-engprod/doa/mobly/cuttlefish/remount_test
Test: run v2/android-gki/adb_remount
Test: atest init#StopServiceByApexName
Flag: EXEMPT bug fix
Change-Id: I22ce8552349ca75b92595b7124d1d06258a7e0c6