Add build.gradle to the code-generation

[jhugman]

This adds the top-level android/build.gradle to the list of files generated with ubrn generate turbo-module command.

Specifically it adds:

    buildFeatures {
      prefab true
    }
    externalNativeBuild {
      cmake {
        cppFlags "-O2 -frtti -fexceptions -Wall -fstack-protector-all"
        arguments '-DANDROID_STL=c++_shared'
      }
    }

to the android.defaultConfig block.