Android O SDK.

This includes changes to:
 build-tools/ -> 26.0.0
 emulator/ (now separate from tools/)
 platforms/ -> android-26
 tools/
 tools-lint/ -> 25.3.2

Bug: 735481
Change-Id: I883ae56aafb1a0158dbfa308bb5ff499732e5c79
Reviewed-on: https://chromium-review.googlesource.com/550585
Reviewed-by: Andrew Grieve <agrieve@chromium.org>