soong: Fix AndroidMk with *Required properties

java.Module is using "Custom" function to write Android.mk.
And if "hostdex" is set to "true", it writes "hostdex" module definition
as well as original module.

As of now, Required/Host_required/Target_required props are filled in
the AndroidMkEntries structure(aosp/939505). But these are not
passed to old AndroidMkData.Custom function.

So, if a java_library declares "hostdex:true" and "required:[...]"
together, "required" is not applied to the "hostdex" variant.

This change copies *Required props from AndroidMkEntries to
AndroidMkData before calling its Custom callback.

Test: m (runs soong unit tests)
Change-Id: I5f85714f721a2a0917ab18072dbea52294c770e7