[Cronet] Add read() replacement that modifies ByteBuffer position() not limit()
This is more in line with other Java APIs (e.g. ReadableByteChannel.read).
Replacing read() directly would be break API compatibility, so this CL adds
a new API, readNew(), to transition embedders to until read() is removed and
readNew() is renamed read() in later CLs. Also included in this CL are some
non-functional changes to use ByteBuffer limit() rather than capacity() when
appropriate.
BUG=531538
Review URL: https://codereview.chromium.org/
1322203004
Cr-Commit-Position: refs/heads/master@{#348868}