Adds app intro component

[ashutoshgngwr]

Changes

• Adds app intro component to explain features to the first-timers
• Fixes a bug (from Fixes save preset button not showing up sometimes #175) that got reintroduced in General Maintenance #311
• Minor code cleanups

Testing

• Tested on a physical device
• Added or modified unit test cases

Others

• Resolves What does timer slider do? #172

[ashutoshgngwr]

Here is an overview of what got changed by this pull request:

Issues
======
- Added 1
           

See the complete overview on Codacy

[codecov]

Codecov Report

Merging #318 into master will increase coverage by 0.95%.
The diff coverage is 97.61%.

@@            Coverage Diff             @@
##           master     #318      +/-   ##
==========================================
+ Coverage   85.76%   86.71%   +0.95%     
==========================================
  Files          29       30       +1     
  Lines        1510     1596      +86     
  Branches      177      178       +1     
==========================================
+ Hits         1295     1384      +89     
+ Misses        115      114       -1     
+ Partials      100       98       -2     

Flag	Coverage Δ
#android	85.97% <97.61%> (+1.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../ashutoshgngwr/noice/sound/player/PlayerManager.kt	84.71% <ø> (+3.11%)	⬆️
...ava/com/github/ashutoshgngwr/noice/MainActivity.kt	78.22% <91.66%> (+0.17%)	⬆️
...hutoshgngwr/noice/fragment/SoundLibraryFragment.kt	86.60% <92.59%> (-1.13%)	⬇️
...com/github/ashutoshgngwr/noice/AppIntroActivity.kt	100.00% <100.00%> (ø)
...thub/ashutoshgngwr/noice/fragment/AboutFragment.kt	99.02% <100.00%> (+0.17%)	⬆️
...ava/com/github/ashutoshgngwr/noice/sound/Preset.kt	93.18% <100.00%> (-2.28%)	⬇️
cast-receiver/src/library.mock.ts	100.00% <0.00%> (ø)
cast-receiver/src/status_ui_handler.ts	97.43% <0.00%> (ø)
... and 1 more

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