Add js variable to check if AAC is successful or not

[swethanagopisetti]

Original AAC PR: #7698

While testing AAC feature, I realized that it would be nice to have a variable whose value reflects the successful start of AAC instead of having to look into the logs. The purpose of this PR is to add such a variable.

More details about this pull request:

1. It adds a javascript variable called "isAACActive" which checks the value of session.active. The value of session.active is set by the successful start of AAC.
   Example:
   let x = nw.Window.get(window); x.enterKioskMode();
   ...
   console.log(x.isAACActive); // It should return True if AAC starts and False otherwise.
2. The value of the variable "x.isAACActive" is set to False by default on non-OSX platforms.

Note: I have a hard time building it to fully test it but I hope that the code is okay and may be it can be tested from the build system's nightly build.

[bluthen]

What work needs to be done for this to be merged. Someone need to do a test build?