[ART] Make killing logd more safe

The current behaviour of 'killall -9 /system/bin/logd' is different for the
master branch and for stable android-9.0.0 branches. For the master branch it
works as expected but for the android-9.0.0 branches it causes devices
to reboot.

This CL replaces 'killall -9 /system/bin/logd' with 'pkill -9 -U logd logd'.

Test: Device with AOSP master, setup-buildbot-device.sh, no 'waiting for device' massages
Test: Device with AOSP android-9.0.0_r6, setup-buildbot-device.sh, no 'waiting for device' messages
Change-Id: I1e33f0c2705c8173c6b29658833d00cf9f345d6f