Simplify the nio Buffer implementations, and fix MappedByteBuffer.

* Fix the reported bug caused by delegation in the MappedByteBuffer
  implementation.

* Collapse the three classes (ReadOnly*, ReadWrite*, and *) used
  to implement most buffer types into one class each.

* Rename HeapByteBuffer to ByteArrayBuffer to match its companions.

* Remove BaseByteBuffer.

* Give the classes used to implement asXBuffer sane names (their
  existing names were backwards).

Bug: https://code.google.com/p/android/issues/detail?id=53637
Change-Id: I2a7ff4f1d502564389615384324afadabccb04f9