CPU Plugin: Prevent integer overflow of total usage

In some cases, the values we read from the system can result in a
negative difference to the previous values. This was already guarded
partially, but only for system/user/wait usage. Total usage would still
be able to go negative, resulting in overflow of the total usage.

Instead, ensure that all the differences are always at least 0, so if we
add things together we aren't adding negatives.

BUG: 448626
FIXED-IN: 5.24.1


(cherry picked from commit f3f509fc)