AndroidTestUtilities: fix cleanup operation

`su` is not available by default on all devices:

> adb shell "echo ls /data/local/tmp | su"
/system/bin/sh: su: not found
> echo $?

Also since `execute_adb_command` use `execute_process` under the hood
the pipeline (`|`) will not be treated as special symbol and will be
process just like one more command line argument for the `adb`.
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