Don't discard ProxyBytesTransferByFgBg

ProxyBytesTransferByFgBg was being handled by an if/else that was
labeled as handing "MobileBytesTransferByFgBg atom or
WifiBytesTransferByFgBg". When refactoring in ag/34015790, I didn't
realize that ProxyBytesTransferByFgBg was handled here, and now it's
getting marked as an "unexpected atom".

Instead, expect the unexpected.

Bug: 437786090
Test: adb shell cmd stats pull-source 10200; adb logcat -s StatsPullAtomService
Flag: EXEMPT BUGFIX

Change-Id: I101537ccf48c011618a488046d606c1787a63886