In mixed builds currently, filegroup doesn't use path prop when creating the paths to the srcs. It defaults to ModuleDir.
Hence, when java.genAidlIncludeFlags [1] calls srcFile.Rel() to eventually create the AIDL include dir for AIDL flags, srcFile.Rel() returns the filepath relative to the module directory instead. This CL appends path prop to module dir when creating relativeRoot.
This fixes the bridge between converted filegroup that set path prop (e.g. libbinder_aidl) to unconverted module (for example, droidstubs). The fix is needed for the child CL aosp/2186599 to convert libbinder_aidl to Bazel. Without this fix, module-lib-api-stubs-docs-non-updatable (unconverted module that depends on libbinder_aidl) can't be built in mixed builds.
[1]: https://cs.android.com/android/platform/superproject/+/master:build/soong/java/gen.go;l=123?q=java%2Fgen.go
Test: go test
Bug: 243010121
Change-Id: Ic2dd2ab9199c62010303a5b8c611d722f4a4118d
