Fix asset encryption and update buildscript

[X1nto]

It seems like I've messed up encryption with my last PR as seen in #95.

While this PR fixes encryption, I also took the opportunity to update the build script to the newest dependencies and managed to clean it up as well.

Changes:

• Use Kotlin's copyTo and use functions for InputStream and OutputStream operations.
• Extract complex build script code to the Util object in buildSrc in order to make it more readable.
• Update AGP, Kotlin, and AppCompat dependencies.
• Use ignoreCase param in shouldEncrypt() function instead of making the joined task list string lowercase.

Fixes:

• Fixed encryption.
• Fixed/Updated README.md to correctly point to ThemerConstants configuration file.

Verified that encryption works on my machine:

Debug APK	Release APK