Make peek*Array methods for Buffers FastNative.

This is step 1 for improving the performance of bulk array getter
methods for Buffer classes which are currently slower than
straightforward loops for small buffers (b/176942060).

Bug: 176942060
Test: m
Test: also roughly estimated time differences with a modified
DirectFloatBufferTest, but saw no new differences.

Change-Id: I958c55ed8099554b48c6732611f5195aa34905fd
2 files changed