Support AllowMissingDependencies in prebuilt_apex modules

Arch-specfic prebuilt_apex modules may be missing prebuilts for new
architectures.  Mark the module as missing dependencies and allow
build analysis to continue when AllowMissingDependencies is set.

Bug: 250918230
Test: lunch aosp_riscv64-userdebug && m ALLOW_MISSING_DEPENDENCIES=true nothing
Change-Id: I79f0c20a0c9443fe443f9ed9bc846aa649f6b6a6
diff --git a/apex/prebuilt.go b/apex/prebuilt.go
index 25ae5bf..0f57911 100644
--- a/apex/prebuilt.go
+++ b/apex/prebuilt.go
@@ -537,7 +537,11 @@
 	}
 
 	if src == "" {
-		ctx.OtherModuleErrorf(prebuilt, "prebuilt_apex does not support %q", multiTargets[0].Arch.String())
+		if ctx.Config().AllowMissingDependencies() {
+			ctx.AddMissingDependencies([]string{ctx.OtherModuleName(prebuilt)})
+		} else {
+			ctx.OtherModuleErrorf(prebuilt, "prebuilt_apex does not support %q", multiTargets[0].Arch.String())
+		}
 		// Drop through to return an empty string as the src (instead of nil) to avoid the prebuilt
 		// logic from reporting a more general, less useful message.
 	}