gdbrunner: take a run-as/su command instead of a user.

Switch from assuming that "root" meant su to an explicit prepended
argument, to allow things like `gdbclient.py --user shell`.

Test: gdbclient.py --user shell -r /system/bin/ls
Change-Id: I9d02eea7c6d79898ed8b13817e009630b0f9536a
2 files changed