Android: Update script and docs for Android Studio

Add conditional check to run `gn gen` if build.ninja does not exist.
This can happen if the out directory is deleted. This way devs won't
have to run `gn gen` manually when `generate_gradle.py` fails.

Update docs for native code editing to start with a lighter target than
`//chrome/browser:browser` and add instructions to manually make
temporary edits to CMakeLists.txt.

Reorder jinja templates to make it easier to see shorter sections.

Bug: 827294,620034
Change-Id: I25be0a67b5f19dc4d1b15e675f0bf1a5beac761c
Reviewed-on: https://chromium-review.googlesource.com/1064696
Commit-Queue: Peter Wen <wnwen@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559671}
4 files changed