Do not register callback for < MIN_THRESHOLD_BYTES

This also avoids returning a nonzero multipath preference
if remaining data is low but nonzero.

Bug: b/72877610
Change-Id: I2459ded06de77d284ae1fe81a60d9e1df56b0c26
Fixes: b/72877610
Test: Tests in go/ag/3842960 pass
3 files changed