Support private tabs for FireFox

[dankeast]

Extra read here:
https://github.com/mozilla-mobile/firefox-android/blob/fenix-v117.0/fenix/app/src/main/java/org/mozilla/fenix/IntentReceiverActivity.kt#L65

Key defined here:
https://github.com/mozilla-mobile/firefox-android/blob/fenix-v117.0/fenix/app/src/main/java/org/mozilla/fenix/HomeActivity.kt#L1290

[MV-GH]

Add this in the else section of the useCustomTab

        if (usePrivateTab) {
            intent.putExtra("com.google.android.apps.chrome.EXTRA_OPEN_NEW_INCOGNITO_TAB", true)
            intent.putExtra("private_browsing_mode", true)
        }

It is supposed to work too if you don't use a customtab

[dankeast]

Add this in the else section of the useCustomTab

        if (usePrivateTab) {
            intent.putExtra("com.google.android.apps.chrome.EXTRA_OPEN_NEW_INCOGNITO_TAB", true)
            intent.putExtra("private_browsing_mode", true)
        }

It is supposed to work too if you don't use a customtab

The option in the menu is "Use private custom tabs if available" so I just followed the current pattern for Chrome. This makes a lot of sense though, so I've changed this to always send the extras if the option is set, and update the option in the menu