Fix memory leak in EnhancedMovementMethod

[khaykov]

Fix

With a help from Leak Canary I found a leak in EnhancedMovementMethod.

In this PR we are using weak reference for a click listener. Task list listener is not leaking, so it's ok to keep it as is.

Test

1. Add link tap handler to aztec instance in Main Activity like this:

 aztec = Aztec.with(visualEditor, sourceEditor, toolbar, this)
            ....
            .setLinkTapEnabled(true)
            .setOnLinkTappedListener(object : AztecText.OnLinkTappedListener {
                override fun onLinkTapped(widget: View, url: String) {
                    val intent = Intent(Intent.ACTION_VIEW)
                    intent.data = Uri.parse(url)
                    startActivity(intent)
                }
            })
            ....

2. Open the app in tap on the link.
3. Confirm that it works and the browser is opened.

Review

@[USER_NAME]

Make sure strings will be translated:

• [ ] If there are new strings that have to be translated, I have added them to the client's strings.xml as a part of the integration PR.