Update avatar image loading logics (PSB-198)

[alfogrillo]

Ticket: https://element-io.atlassian.net/browse/PSB-198

Description
This PR makes the avatar image loading more robust:

• Adding multiple fallback images
• Setting the fallback before the avatar url is fetched to avoid leaving a blank image view

Fixes: #6847

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[codecov]

Codecov Report

Base: 54.86% // Head: 11.76% // Decreases project coverage by -43.09% ⚠️

Coverage data is based on head (5043ece) compared to base (6b27303).
Patch coverage: 8.20% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop    #6927       +/-   ##
============================================
- Coverage    54.86%   11.76%   -43.10%     
============================================
  Files          423     1594     +1171     
  Lines        16607   156625   +140018     
  Branches      6743    63252    +56509     
============================================
+ Hits          9111    18431     +9320     
- Misses        7288   137571   +130283     
- Partials       208      623      +415     

Flag	Coverage Δ
uitests	54.88% <55.00%> (?)
unittests	6.15% <1.64%> (?)

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

Impacted Files	Coverage Δ
Config/BuildSettings.swift	43.07% <ø> (+43.07%)	⬆️
Riot/Modules/Common/Avatar/AvatarView.swift	35.29% <0.00%> (ø)
Riot/Modules/Common/Avatar/AvatarViewData.swift	0.00% <0.00%> (ø)
...Modules/Common/Avatar/AvatarViewDataProtocol.swift	0.00% <0.00%> (ø)
Riot/Modules/Room/CellData/RoomBubbleCellData.m	0.00% <0.00%> (ø)
...ngs/UIKit/RoomNotificationSettingsAvatarView.swift	0.00% <0.00%> (ø)
Riot/Modules/Room/RoomCoordinator.swift	0.00% <0.00%> (ø)
Riot/Modules/Room/RoomViewController.m	0.00% <0.00%> (ø)
...lls/Styles/Bubble/BubbleRoomTimelineCellProvider.m	0.00% <0.00%> (ø)
...st/Incoming/VoiceBroadcastIncomingBubbleCell.swift	0.00% <0.00%> (ø)
... and 1221 more

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[gileluard]

lgtm