Only set fullIndex flag when performing a full update

Previously UpdateData#fullIndex was default true which meant if a
partial update came in before a call to Index#update() then the
partial update would be treated as a full index and future calls
to Index#update() would be ignored.

Bug: 30366360
Change-Id: I6e2738c25c9623f49171ecba0f3e0ce0076b7e42