Block files without trailing newlines

Add a pre-submit check to ensure that files have a newline character at
the end.

Please see https://android.googlesource.com/platform/tools/repohooks/
for documentation on how PREUPLOAD hooks work.

Test: created a change and watched the presubmit check reject it.
Change-Id: Id0528cb1bd6fa9c4483ba43720839832f4fec34d