Remove erroneous parameter to fix buildDiffWrapper

In ag/828480, RawContactDelta.buildDiffWrapper() was created by copying
buildDiff() in the same class. The parameter "firstIndex" was copied by
mistake and has been removed in this CL.

Bug 25629359

Change-Id: Ibf6bd2414148b5cdcf4cf3002bc8d32daa234e31