Explicitly set generate_hashtree: false

Non-updatable APEXes are going to be only present on a dm-verity
protected read-only partition. For such APEXes there is no need in
embedding a hashtree.

This is a no-op change in preparation of switching default value of
generate_hashtree to true.

Bug: 190621617
Test: m
Change-Id: Ib9ceb38f316a06e4a39750adb94561c368b9d6f6