Upgrade Billing library to 6.0.1

[TranceLove]

Also

• some code cleanups at Billing class
• move package namespace to build.gradle

Cannot build the play release to test IAP functions yet, so may probably did it in the blind. Comments welcome. Fixes #136

[VishalNehra]

Getting following crash for subscriptions. Non subscription lifetime purchases seem to work fine

Issue explanation (write below this line)

Exception

• App Name: File Utilities
• Package: com.amaze.fileutilities
• Version: v1.91
• User Action: UI Error
• Request: Application crash
• OS: Linux Android 13 - 33
• Device: OnePlus9R
• Model: LE2101
• Product: OnePlus9R_IND
• Device ID: a4bb03819251471b3bd7679653c3a2a8a385fdb499708856cd1d71760b9b9b25
  Crash log

java.lang.NullPointerException: offerToken is required for constructing ProductDetailsParams.
    at com.google.android.gms.internal.play_billing.zzm.zzc(com.android.billingclient:billing@@6.0.1:8)
    at p3.d.onClick(R8$$SyntheticClass:82)
    at android.view.View.performClick(View.java:7570)
    at android.view.View.performClickInternal(View.java:7540)
    at android.view.View.-$$Nest$mperformClickInternal(Unknown Source:0)
    at android.view.View$PerformClick.run(View.java:29710)
    at android.os.Handler.handleCallback(Handler.java:942)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loopOnce(Looper.java:240)
    at android.os.Looper.loop(Looper.java:351)
    at android.app.ActivityThread.main(ActivityThread.java:8427)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:584)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1013)

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