Share EffectiveLicenseFiles for snapshot modules

License files must be captured with the snapshot modules. NoticeFiles
does not have the information provided by the `license` module.
Reading the `Effective_license_text`, snapshot includes the required
license files.

Bug: 190690041
Test: build
      check the NOTICE files built from snapshots
Change-Id: I96c4579b4bef484cbb39fc06c707742a02a2b90b

cc/linkable.go

@@ -88,6 +88,9 @@ type Snapshottable interface {
 	// SnapshotLibrary returns true if this module is a snapshot library.
 	IsSnapshotLibrary() bool
 
+	// EffectiveLicenseFiles returns the list of License files for this module.
+	EffectiveLicenseFiles() android.Paths
+
 	// SnapshotRuntimeLibs returns a list of libraries needed by this module at runtime but which aren't build dependencies.
 	SnapshotRuntimeLibs() []string