Add 29 to stubs.versions of libc/libdl/libm

Stubs of version 29 should be provided to those APEX modules targeting
previous SDK release.

Bug: 145796956
Test: m
Change-Id: I9454fbf81377aba25e75a6fdfc77cbb070eaadde
diff --git a/libc/Android.bp b/libc/Android.bp
index 80b157a..650cfbd 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1696,7 +1696,10 @@
 
     stubs: {
         symbol_file: "libc.map.txt",
-        versions: ["10000"],
+        versions: [
+            "29",
+            "10000",
+        ],
     },
 
     apex_available: [