fix(firebase-analytics): consistent handling of Podfile variable $NSFirebaseAnalyticsWithoutAdIdSupport

[edusperoni]

Some context:

Before we would find the platforms/ios/Podfile and add $NSFirebaseAnalyticsWithoutAdIdSupport to the top. Turns out that the file is not always available on before-checkForChanges and it's only available consistently in after-prepare, which is already too late (pods already installed). As a result, the first build would not have the option, and from the second build onwards it would.

So instead of adding another hook to the cli, I simply modify the Podfile in node_modules. The code is resilient enough to always update the Podfile if the desired result is different from the current state (if $NSFirebaseAnalyticsWithoutAdIdSupport=true in the user Podfile, it will add to the plugin Podfile if missing. If NSFirebaseAnalyticsWithoutAdIdSupport=true is present in the plugin Podfile but not on the user Podfile then we always remove it).

It also now supports comments (#$NSFirebaseAnalyticsWithoutAdIdSupport=true will not add to the podfile).

Ideally we should fix the CLI so we could have a BeforePluginsPodfile (or something. Similar to before-plugins.gradle on android)

Fixes #128