Native API feedback for puller API

1. Rename registerPullAtomCallback to setPullAtomCallback
2. Rename unregisterPullAtomCallback to clearPullAtomCallback
3. Change Ns to Millis for consistency with java
4. Swap metadata and callback params in setPullAtomCallback to match
java
5. Added getters for PullAtomMetadata
6. Added libstatspull_test to test PullAtomMetadata
7. Changed the types on setAdditiveFields from int to int32_t

Test: make
Test: atest LibStatsPullTests
Test: bit libstatspull_test:*
Bug: 1507885621
Bug: 151875223
Change-Id: I5cb07bfe71b1002180403828d5e8e6a5b88ce6fe