Cleanup nativeHeapZeroInit and memtagMode implementation.

* Rename nativeHeapZeroInit to nativeHeapZeroInitialized.
* nativeHeapZeroInitialized is a tri-state, so make it an @IntDef.
* Add documentation to some accessor methods, and missing @type
  annotations.
* Add attrs_manifest.xml document for the new attributes.

Bug: 182165383
Bug: 182370211
Test: com.android.cts.tagging
Change-Id: Iad46a1da9bc040baf4c438afbc5fd8cd128e99f1