SDK / exoplayer

1. Fix SimpleCache startReadWrite asymmetry. Allow more concurrency.

- startReadWrite does not have the concept of a read lock. Once
  a cached span is returned, the caller can do whatever it likes
  for as long as it wants to. This allows a read to be performed
  in parallel with a write that starts after it.
- If there's an ongoing write, startReadWrite will block even if
  the return operation will be a read. So there's a weird asymmetry
  where reads can happen in parallel with writes, but only if the
  reads were started first.
- This CL removes the asymmetry, by allowing a read to start even
  if the write lock is held.
- Note that the reader needs to be prepared for the thing it's
  reading to disappear, but this was already the case, and will
  always be the case since the reader will need to handle disk
  read failures anyway.

2. Add isCached method.