docs: Add more troubleshooting info for build and run.

[agrawal-d]

Adds command to clean gradle build, and link to upstream
troubleshooting docs.

[chrisbobbe]

Merged, thanks!

I just made one small tweak: changing gradlew clean to ./gradlew clean. My understanding is that sometimes the shell needs a hint that you want it to run a script in a particular file, as opposed to a built-in command or something in the PATH variable — the ./ at the beginning makes it clear that it's a file path. If you didn't cd into android first, you could say android/gradlew, without the leading ./, because it already looks enough like a file path that it's reasonable to assume it is one.

[chrisbobbe]

And without the leading ./, I get this:

$ gradlew clean
bash: gradlew: command not found