toolbox: add prlimit command

adb shell prlimit <pid> <cur> <max>

prlimit uses prlimit internally to allow you to set the limits on an arbitrary
process, if you have the privileges to.

Change-Id: Ie9a5dc510c7c695debd8f948361d4c51befa5604
Signed-off-by: Iliyan Malchev <malchev@google.com>