Fix: 'required' property is ignored

This change fixes a bug that the 'required' property of a module is
ignored when the module is using a shared from an APEX. This is
happening because LOCAL_REQUIRED_MODULES is overwritten (with := instead
of +=) when ApexesProvidingSharedLibs is not empty.

Fixing the bug by appending ApexesProvidingSharedLibs to
AndroidMkData.Reuired so that it is handled in android/androidmk.go

Test: m
Test: build with https://android-review.googlesource.com/c/platform/bionic/+/849044
and make sure that system/bin/vold_prepare_subdirs exist.

Change-Id: Ie57bca480ba4198b4da0df1c73e92fa42b5ebda8