Support using api-versions.xml from another module

Metalava has two different flags surrounding api-levels:
- one for generating api-versions.xml to a file
- one for applying api-versions.xml from a file

Previously, soong always applied both of these arguments at the same
time, such that framework-doc-stubs both generated and applied
api-versions.xml.

Add support for using api-versions.xml from another module name as well.

Bug: 187398174
Test: droidstubs_test.go
Change-Id: I8288fe4788336d5d5c60d09d48b00ca111449fba