Puller refactor

1) Refactor pullers and add tests.

2) Add timeout to a puller.
mPullTimeoutNs is intrinsic to puller. A pull taking longer than this is
deemed failed and the data discarded.
A metric or StatsPullerManager requesting a pull should monitor the pull
and have deadlineNs. A successful pull may come later than desired due
to statsd processing delays.

3) Add unit tests to puller now that the base puller is more
complicated.

Bug: 118756964
Test: unit test
Change-Id: I0e5d47e2527391f7beef4b2d06bfd5c2f82f1179