Reformat java.nio.Buffer impls for update to 8u121-b13.

This CL integrates (mostly) javadoc reformatting changes
from upstream OpenJDK8u121-b13 for some java.nio.*Buffer
classes. Behavior changes and Android change markers are
left for a future CL. This CL also updates the copyright
years to be those of 8u121-b13.

The upstream classes are generated from the same
template but separate in Android.
 - The upstream classes have a lot of redundant empty lines.
   Most of those are not kept on Android.
 - The decision which redundant empty lines are kept (and
   perhaps also minor other formatting changes) may not
   be 100% consistent between the files in this CL.

The following is left for a future CL to investigate:
- upstream's put() has logic for throwing ReadOnlyBufferException.
  Needs to be looked at for a follow-up CL. It may be related to
  this Android change in CharBuffer:
  "Android-changed: Throw ReadOnlyBufferException as soon as possible."
- Extra super ctor argument for Buffer._elementSizeShift is missing
  a change marker. Note that this area is reformatted by this CL.
- *Unchecked() methods added in Android libcore commit
  e20e1b55 need change markers.
- Need to look into diff in the following changes to figure out
  whether they are Android vs. upstream changes and whether they
  change behavior:
   - ByteBuffer.allocateDirect(int)
   - ByteBuffer.put(ByteBuffer).
   - CharBuffer.chars()
- Need to check whether DoubleBuffer.compareTo() change is
  a bug fix ("duplicating pointlessly" suggests it's only a
  refactoring?).
- Other (generally indirect) subclasses of java.nio.Buffer,
  e.g. DirectByteBuffer or ByteBufferAsCharBuffer, were not
  considered.

A typo in the updated upstream javadoc for bulk get()
methods is fixed (the typo is not taken) in this CL:
upstream javadoc contains "src.get():" in a code snippet,
which should be "src.get();".

Bug: 35910877
Test: libcore/tools/upstream/upstream-diff \
      java.nio.{Byte,Char,Double,Float,Int,Long,Short}Buffer
Change-Id: I50c3abe0951946223e26cd38abd8caa0b3b73d60