pixelstats: add readproc to access process

We would like to collect stime of kswapd0 and kcompactd0.
readproc is required to access /proc/<pid>

Bug: 173101018
Test: build pass
Test: adb shell cmd stats print-logs
      adb logcat | grep 105014
      stime can be collected.
Change-Id: I1beb6e1a4b9ea04b610bba88b649e21cb0b236a8