Enable android_app_import to supply framework-res.

By enabling an android_app_import module to supply the
special framework-res module, it becomes possible to
create a split build that excludes the source of over
10,000 files under frameworks/base/core/res, replacing
this source with a prebuilt. However, since other parts
of a monolithic build use the source of framework-res,
this scheme works only if your split build manifest
excludes modules that depend on framework-res source.

Bug: 176851898
Test: lunch aosp_arm64-userdebug && m nothing
Test: new TestAndroidAppImportExportPackageResources
Test: local build without prebuilt framework-res
Test: local build with prebuilt framework-res
Change-Id: If00d9dbc656c8406c038448d09f93f767c1e06d1