Disable screen-off RPM timing in BatteryStats

RPM = resource power manager.
Fetching the rpm stats (specifically, the subsystem low power stats) is
slow... too slow to reasonably do each time the screen state changes.
Therefore, it is disabled until fetching this information can be done
more quickly. Consequently, the screen-off RPM values will be incorrect
until it is re-enabled; they are therefore not printed.

Bug: 65164435
Bug: 62549421
Test: manual
Change-Id: I54d54f244c69ee372f22ecd99f32570db4aeb222