Use dot instead of colon as separator of custom part.

This is done becase ':' is already used as a separator between package
and type in a full resource name. So if we use ':' it will be consfusing
for custom resource types: com.my.package:layout:2/myLayout ->
com.my.package:layout.2/myLayout.

DD: go/custom-resource-types-in-aapt2

Bug: b/215108200
Test: Resource_test
Change-Id: Ieed256e4376abe0a3788d8198110bef189b55740