Support properties in path when merging overlay configs

Make it possible to use read only properties in the path when merging
an overlay config.

One of the usecases is to support switching overlays depending on SKU.

Example:
<config>
  <overlay package="..." enabled="true" mutable="false" />
  <merge path="${ro.my.prop}/config.xml" />
  <merge path="skus/${ro.my.prop}.xml" />
  <overlay package="..." enabled="true" mutable="false" />
</config>

Bug: 223913150
Test: atest OverlayConfigTest OverlayConfigParserTest
Change-Id: Ic9e460e5064a9601e713b3959a66a80bdf78734a