Parcel: obtain(IBinder) as API

There are two main usecases for this:
- provide an alternative route to implement features like markSensitive
  (which requires only 1 function call per class, rather than a
    function call and flag per transaction type)
- support RPC format transactions

Bug: 175814583
Test: atest android.os.ParcelTest, boot, manual
Change-Id: Ibaacc27a125a31e18d4b5889d05d12cc18d05624

core/api/current.txt

@@ -29976,6 +29976,7 @@ package android.os {
     method public boolean hasFileDescriptors();
     method public byte[] marshall();
     method @NonNull public static android.os.Parcel obtain();
+    method @NonNull public static android.os.Parcel obtain(@NonNull android.os.IBinder);
     method @Nullable public Object[] readArray(@Nullable ClassLoader);
     method @Nullable public java.util.ArrayList readArrayList(@Nullable ClassLoader);
     method public void readBinderArray(@NonNull android.os.IBinder[]);

core/java/android/os/Parcel.java

@@ -454,8 +454,6 @@ public final class Parcel {
      * transaction on this specific binder object. Based on this, the format of the wire binder
      * protocol may change. For future compatibility, it is recommended to use this for all
      * Parcels.
-     *
-     * @hide
      */
     @NonNull
     public static Parcel obtain(@NonNull IBinder binder) {