Co-authored-by: DimaG <dgeorgiev06@gmail.com>

Provide a default and builtin default implementation which finishes the
Activity, same as before.

What this PR removes is the ability to provide a custom default handler because
applications can already take this decision when calling `onBackPressed`. In
addition, make `onBackPressed` return `void` because it's virtually impossible
for it to return `false` (that would mean that there is no
`ReactInstanceManager`, which means there is no app to begin with).

In addition, remove the use of `BackAndroid` since `BackHandler` contains an iOS
shim now.

This is done at the app level, not the SDK.

Currently 2 Firebase services are used:

  - Crashlytics
  - Dynamic Links

They are enabled in tandem, if the appropriate Google services file
(GoogleService-Info.plist on iOS or google-services.json on Android) is found.

Each service needs to be individually enabled in the Firebase console.

The annotation processor is required for our Glide module to be included.

Note that Android 9 Pie (API 28) disallows HTTP requests by default, so an
exception was needed in the app in order for the Metro bundler to work in debug
mode.

The Google repository is already added with google().

Glide (which is used by react-native-fast-image) can cause trouble if the host
app (the one using the SDK) is using Glide already.

To avoid this, don't use the builtin AppGlideModule (as the docs recommend) and
let apps define it.

Every time the android version that is used changes we need to update the license hash.

The dependency was dropped, so it's no longer needed.

Assume it's at the same level as the Jitsi Meet repo, by default.

Set them to the next release versions. In additon, the buildNumber variable will
be used to match the requirements of versionCode:
https://developer.android.com/studio/publish/versioning

that is, a monotonically increasing number, independent of the app / sdk
version.

The upstream package has been unmaintained for 2 years now, and making the litle
changes needed as React Native needs them is getting old. The actual
funcionality is a couple of one-liners plus tons of boliterplate, which gets
reduced by quite a bit if we just embed it. So here it goes.

Due to a switch to a newer version of JSCore, the jsc-android dependency is now used by the
SDK. As this dependency is not (yet) available in the Jitsi Maven repository, an error like
this is reported when an application is ran that uses the SDK:

    com.facebook.react.common.JavascriptException: Can't find variable: Symbol

This commit primarily improves the instructions on how to create a local Maven repository
that contains all required dependencies, including the JSCore dependency that was missing.

This intends to address the issue described in https://github.com/jitsi/jitsi-meet/issues/3399

Since this is a feature implemented in the SDK, it makes sense that all the
plumbing required to make it work it's in the SDK itself.

This makes the PermissionsAndroid builtin module work.

Introduce the JitsiMeetActivityInterface, which defines the interface that
activities using JitsiMeetView directly must implement in order to ensure full
functionality.

LeakCanary is a memory leak detection library which will run only in Debug mode.

The JSC version used by React Native is about 3 years old, and doesn't implement
things like Symbol or Typed Arrays, which require polyfills. These polyfills are
sometimes a los less performant, as is the case for Typed Arrays.

Bumping an updated JSC version makes both platforms consistent when it comes to
the JavaScript platform.

Use react-native-fastimage, which uses 2 full-native image impleentations using
well known and mature (native) libraries.

This gets us rid of 2 libraries which were observerd as a source of bugs and
created trouble with dependencies: react-native-fetch-blob and
react-native-img-cache. They are also no longer well maintained.