Add animation on the timeline content, to avoid glitch (#1323)

[bmarty]

Type of change

• Feature
• Bugfix
• Technical
• Other :

Content

Workaround the load of the timeline by delayed the display to 150ms and passing from 0 to 1 for alpha in 150ms.
This reduce the glitches, and is independant of the loading of the messages behind the scene.

Motivation and context

Fixes #1323

Screenshots / GIFs

Tests

• Open a timeline and see there is less glitch

Tested devices

• Physical
• Emulator
• OS version(s):

Checklist

• Changes have been tested on an Android device or Android emulator with API 23
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/vector-im/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request includes a new file under ./changelog.d. See https://github.com/vector-im/element-x-android/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR

[sonarqubecloud]

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[codecov]

Codecov Report

Patch coverage: 52.94% and project coverage change: -0.01% ⚠️

Comparison is base (3ff7c62) 58.06% compared to head (1411b7d) 58.06%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1405      +/-   ##
===========================================
- Coverage    58.06%   58.06%   -0.01%     
===========================================
  Files         1128     1129       +1     
  Lines        29863    29879      +16     
  Branches      6060     6067       +7     
===========================================
+ Hits         17341    17349       +8     
- Misses        9930     9932       +2     
- Partials      2592     2598       +6     

Files Changed	Coverage Δ
...libraries/designsystem/animation/AlphaAnimation.kt	46.66% <46.66%> (ø)
...id/features/messages/impl/timeline/TimelineView.kt	44.51% <100.00%> (+0.34%)	⬆️

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[ganfra]

I don't get the LaunchedEffect, how is it needed?

[bmarty]

This is a way to pass the alpha from 0 to 1.

[bmarty]

As you suggested, it could have been implemented like this:

    var isVisible by remember { mutableStateOf(false) }
    LaunchedEffect(Unit) {
        isVisible = true
    }
    val alpha by animateFloatAsState(
        targetValue = if (isVisible) 1f else 0f,
        animationSpec = tween(
            delayMillis = 150,
            durationMillis = 150,
        ),
        label = "label"
    )

We may iterate later.