Update checkapi error message to provide more guidance

With from-text stub feature, checkapi is also used for checking the
sanity of the stubs. Therefore, this change adds more explanatory
message about how the user can rebuild the target when their build
failed with stub validation after local API change.

Test: m nothing
Bug: 309027703
Change-Id: If7c9c287266f3da82edde663619b9f08ee05530d

java/droidstubs.go

@@ -775,6 +775,11 @@ func (d *Droidstubs) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 			`         m %s-update-current-api\n\n`+
 			`      To submit the revised current.txt to the main Android repository,\n`+
 			`      you will need approval.\n`+
+			`If your build failed due to stub validation, you can resolve the errors with\n`+
+			`either of the two choices above and try re-building the target.\n`+
+			`If the mismatch between the stubs and the current.txt is intended,\n`+
+			`you can try re-building the target by executing the following command:\n`+
+			`m DISABLE_STUB_VALIDATION=true <your build target>\n`+
 			`******************************\n`, ctx.ModuleName())
 
 		rule.Command().