extract_utils: Seperate multiple override targets with comma

This follows the style google uses, for example in [1].

With this change it will generate 'overrides: ["TargetOne", "TargetTwo"],'
instead of 'overrides: ["TargetOne TargetTwo"],' for this line:
-system/app/TargetZero/TargetZero.apk;OVERRIDES=TargetOne,TargetTwo

[1]: https://github.com/LineageOS/android_packages_apps_CellBroadcastReceiver/blob/2daf3ecf897c35725c8ae78f7fd6533773a8bf50/Android.bp#L53

Change-Id: I154ee3a4cc51e0938c860dcfc125eacf279d102e
1 file changed