Android billing security issue

I plan to use billing in the application to enable some functions after payment by the user. I would like to take the following steps:

• Follow the Google example to add in-app purchases to it.
• After the user has paid, add the Boolean flag to the private SharedPreferences.
• Do not double-check whether the user pays or not.

I would like to know more about the security of billing in the application. My question is:

• This is a simple crack (just download the program and execute it) for the Android licensing library, is this a similar problem for billing in the application?
• Are private sharedPrefences safe enough? hashing a boolean variable seems useless?
• Is it easy to crack the apk file, for example, just decompose apk, find the logical logic to 1> 0, i.e. always true and then recompile apk?

I found that there is AndroidBillingLibrary for easy implementation, but it is deprecated. Is this a good alternative?