Docs: consistent fenced code blocks

No change to logic, only docs. This is a mechanical docs change which
should not impact content (but will improve readability).

This edits some Android docs to use fenced code blocks more
consistently:

 * Prefer fenced code blocks over indented code blocks
 * Fix indentation issues (see coverage.md)
 * Specify a language for syntax highlighting where it makes sense
   (particularly, use the "gn" language where it was omitted)

R=agrieve@chromium.org

Bug: 918221
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I2a4aa292d3bd1cf67bd0a7bcfb0cb01f7b8ede98
Reviewed-on: https://chromium-review.googlesource.com/c/1451357
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628981}
5 files changed