fix: prebuilt_etc_xml

Since aosp/872653 and aosp/904233 landed, prebuilt_etc_xml has been broken.
aosp/872653: Enable arch variant properties in prebuilt_etc.
aosp/904233: Add a prebuilt module type for usr/share.

This change fixes
1) pass baseDir "etc" to initialize PrebuiltEtc struct
2) change "multlib" argument to MultilibFirst to reflect aosp/872653

Bug: 138082739
Test: m com.google.vr.platform.xml && see if it is installed in /system/etc
Change-Id: I4802107fd8ccf28a1170d7d165700c92df32e341