misc/androidstudio: check on javaCompile to avoid exception

[chiaen]

In commit https://git.io/vQ1gV, the classpath was delayed evaluated to
the task executions. But at Library mode, there will be exception
because the javaCompile involved is not intialized. Add check to fix it.

[gopherbot]

Hi! Thanks for the PR!

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GopherBot (on behalf of the Go Team)