Address Puller API Feedback

1. Rename registerPullAtomCallback to setPullAtomCallback
2. Rename unregisterPullAtomCallback to clearPullAtomCallback
3. Add getters to PullAtomMetadata
4. Change Ns to Millis (when I tried to make it Nanos, I received a
built time error saying to prefer millis unless we need the precision.
We do not need the precision, so I changed it).
5. Fix out of order params.

I did not change usePooledBuffer to setPooledBuffer because I think use
is more appropriate for our use case.

Test: make
Test: atest PullAtomMetadataTest
Test: atest GtsStatsdHostTestCases
Bug: 149475498

Change-Id: Ib07aa57a6e02c77917fe0e65a3d4a77c00ce8565