Android: Default to Android Studio's SDK

Enabling the default as per: https://crrev.com/c/1058308

Obviates the need to pass in `--sdk AndroidStudioDefault` and making
local edits to //third_party/android_tools/sdk when updating the sdk.

Update docs so that using the `gradle` folder when importing is more
prominent.

Bug: 620034,843264
Change-Id: I3c3c56a7d15b57ce0aaa9d4029493dc574045ceb
Reviewed-on: https://chromium-review.googlesource.com/1070453
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561184}
2 files changed