[SM15] Fix MobileBytesTransfer puller get lesser NetworkStats

Currently, puller of MobileBytesTransfer(ByFgBg) are collecting
NetworkStats since boot. Since history of network stats only
have 2 hours of resolution, when it is being queried, service
will assume that history network stats has uniform
distribution and return a fraction of network stats that is
originally subject to 2 hours. Hence, less NetworkStats will be
reported if device is booted within 2 hours.

This doesn't impact usual metrics collecting since the time
between boot and pull for metrics is usually long. But it will
impact the tests given that they are usually run just after boot.

Test: adb shell cmd stats pull-source 10000~10003
      adb shell dumpsys netstats --uid
Test: atest android.cts.statsd.atom.UidAtomTests#testMobileBytesTransfer
Test: atest android.cts.statsd.atom.UidAtomTests#testMobileBytesTransferByFgBg
Bug: 153174569
Change-Id: I9459dace8734d9bf61f9828b74b3dc58c03940e8