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