ProcessList: use different polling interval when screen off

We are using the same time interval for both screen off and screen on.
For process on top: ActivityManager.PROCESS_STATE_TOP, we will poll PSS
information every 120 sec.
During polling PSS, we are grep /proc/%d/smaps which is a 10k lines file.
This cost quite a lot of CPU resource. But when screen off, we don't need to update
TOP process PSS so frequently since we are not operating it. Lower polling
circle to every 15min to reduce power.

Change-Id: Ia4cd26290d44d066f034d5270ce5523650a3c493