Enable Shrinking , Obfuscate and App Optimization as per best practices

[aakash1313]

Will pick this after finishing work manager.

[justinklchan]

@aakash1313 when you are able, please add a brief description of what this entails, thanks.

[aakash1313]

@justinklchan : This basically covers everything discussed here : https://developer.android.com/studio/build/shrink-code