Error when WITHOUT_CHECK_API=true in from-text stub build

Given that WITHOUT_CHECK_API=true is incompatible with from-text stub
build, this change makes the module raise an error when the two
conflicting configurations are set.

This change also modifies the checkapi error message, that
DISABLE_STUB_VALIDATION does not bypass checkapi.

Test: m nothing --no-skip-soong-tests && WITHOUT_CHECK_API=true m nothing
Bug: 322060779
Change-Id: I597309baa8fcfbefa05863c7b392bfb8b6774cea
diff --git a/java/droidstubs.go b/java/droidstubs.go
index 0626313..0460265 100644
--- a/java/droidstubs.go
+++ b/java/droidstubs.go
@@ -1194,7 +1194,8 @@
 			`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`+
+			`m DISABLE_STUB_VALIDATION=true <your build target>.\n`+
+			`Note that DISABLE_STUB_VALIDATION=true does not bypass checkapi.\n`+
 			`******************************\n`, ctx.ModuleName())
 
 		rule.Command().