Prevent estimates from updating while callbacks are happening

It seems that for some people the estimate was being set to null
as part of the process to update the estimate as they were trying
to access it. This CL makes it so that we don't allow callbacks
to start if we are currently updating which will ensure that this
doesn't happen again.

Test: repro'd issue without change, issue gone with change
Bug: 131607084
Change-Id: I938d05a3977bb4bba2c5d28e800d94120b8425d2