Use 'kill' to kill the system_server process

Somehow 'pkill' failed to kill the system_server process and failed the
test. Let's use 'pidof' + 'kill' as a workaround.

Bug: 151947348
Test: atest apex_rollback_tests
Change-Id: I5d16097f2827f5d8ab35822d093985b51268e044
1 file changed