Add ForwardingOs to CorePlatformApi.

The class was already annotated @CorePlatformApi, but that had
no effect because ForwardingOs.java was in non_openjdk_java_files.
This CL moves it to non_openjdk_javadoc_files and marks it @hide,
which has the effect of adding it to CorePlatformApi.

Test: Manually checked that frameworks code referring to ForwardingOs
      does now compile.
Bug: 118100586

Change-Id: I2e0d88fec15cf20e119e1d1bffed98b5937babf1
diff --git a/non_openjdk_java_files.bp b/non_openjdk_java_files.bp
index bf17bd1..4513c47 100644
--- a/non_openjdk_java_files.bp
+++ b/non_openjdk_java_files.bp
@@ -169,6 +169,7 @@
         "luni/src/main/java/libcore/internal/StringPool.java",
         "luni/src/main/java/libcore/io/DropBox.java",
         "luni/src/main/java/libcore/io/EventLogger.java",
+        "luni/src/main/java/libcore/io/ForwardingOs.java",
         "luni/src/main/java/libcore/io/IoBridge.java",
         "luni/src/main/java/libcore/io/IoUtils.java",
         "luni/src/main/java/libcore/io/Libcore.java",
@@ -299,7 +300,6 @@
         "luni/src/main/java/libcore/io/ClassPathURLStreamHandler.java",
         "luni/src/main/java/libcore/io/BlockGuardOs.java",
         "luni/src/main/java/libcore/io/BufferIterator.java",
-        "luni/src/main/java/libcore/io/ForwardingOs.java",
         "luni/src/main/java/libcore/io/IoTracker.java",
         "luni/src/main/java/libcore/io/Linux.java",
         "luni/src/main/java/libcore/io/MemoryMappedFile.java",