libc: Add stubs.versions=R

Add version R to libc so that APEXes can rely on version R stubs of
libc.

Bug: 152655956
Test: m
Merged-In: I70d17702d9885760af658e23ac6e3106405ff3bc
Change-Id: I70d17702d9885760af658e23ac6e3106405ff3bc
(cherry picked from commit 75e79c92dcd6c62f2e350898f813ba729b0e032e)
diff --git a/libc/Android.bp b/libc/Android.bp
index 987446d..a969edd 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1700,6 +1700,7 @@
         symbol_file: "libc.map.txt",
         versions: [
             "29",
+            "R",
             "10000",
         ],
     },