hiddenapi: Initialize unused value in DexFile::MapItem

The build tool adds a new MapItem into the MapList of a dex file
which points to the new hiddenapi flags section. However, hiddenapi
would not initialize the `unused_` field in MapItem and therefore
result in non-deterministic builds.

Bug: 119308882
Test: invoke hiddenapi twice of the same dex, diff the two files
Change-Id: I00d0dc909a3424f3eb2776cf87719c074f264e74