Fix build scripts on Mac OS/Linux

[jschool504]

Steps to reproduce

1. Attempt to run npm run build:android on Mac OS or Linux

Expected
App should build successfully.

Actual
Error indicating that gradlew command cannot be found is thrown.

Resolution
Probably the best solution to this is to create a build script that detects the OS and uses the correct call (gradlew on Windows, ./gradlew anywhere else) to build the app.

[jschool504]

NOTE: deployment server currently works around this issue by changing the gradlew command locally and using git pull --rebase to reapply that change to any updates. This workaround must be removed before this issue is merged in, otherwise the build system will break.