Refactor puller of {Mobile|Wifi}BytesTransfer(ByFgBg)

This is a no-op refactoring that minimize the change needed
to add new puller of supporting data usage per rat type atom
in the follow-up patches.

(CP of CL from master to rvc-dev)

Bug: 129082217
Test: adb shell cmd stats pull-source 10000~10003
Test: atest android.cts.statsd.atom.UidAtomTests#testMobileBytesTransfer
Test: atest android.cts.statsd.atom.UidAtomTests#testMobileBytesTransferByFgBg

Change-Id: If544dd5a1dfc10bc70bf381f79a938f7d6b1dcb7
Merged-In: If544dd5a1dfc10bc70bf381f79a938f7d6b1dcb7