Allow apex to use generated file_contexts

To support trunk stable, file_contexts files should be processed before
creating APEX images. That means that file_contexts may be under out/.
This change relaxes the path check so we can use Soong-generated
file_contexts, not only the real source file_contexts. Of course the
restriction that modules should be under system/sepolicy is still
enforced.

Bug: 306563735
Test: build with generated file_contexts; see virt apex
Change-Id: I3aa368b338141d4826fc8622e1c8d2838bfd8484
diff --git a/apex/builder.go b/apex/builder.go
index ba4df5f..8ec07fa 100644
--- a/apex/builder.go
+++ b/apex/builder.go
@@ -344,10 +344,12 @@
 func (a *apexBundle) buildFileContexts(ctx android.ModuleContext) android.OutputPath {
 	var fileContexts android.Path
 	var fileContextsDir string
+	isFileContextsModule := false
 	if a.properties.File_contexts == nil {
 		fileContexts = android.PathForSource(ctx, "system/sepolicy/apex", ctx.ModuleName()+"-file_contexts")
 	} else {
 		if m, t := android.SrcIsModuleWithTag(*a.properties.File_contexts); m != "" {
+			isFileContextsModule = true
 			otherModule := android.GetModuleFromPathDep(ctx, m, t)
 			fileContextsDir = ctx.OtherModuleDir(otherModule)
 		}
@@ -363,7 +365,7 @@
 			ctx.PropertyErrorf("file_contexts", "should be under system/sepolicy, but found in  %q", fileContextsDir)
 		}
 	}
-	if !android.ExistentPathForSource(ctx, fileContexts.String()).Valid() {
+	if !isFileContextsModule && !android.ExistentPathForSource(ctx, fileContexts.String()).Valid() {
 		ctx.PropertyErrorf("file_contexts", "cannot find file_contexts file: %q", fileContexts.String())
 	}