[FIX] Empty DMs go to sidebar's top after updating user preferences

[matheusbsilva137]

Proposed changes (including videos or screenshots)

• Do not use _updatedAt as lm field when merging info from subscriptions and rooms (since this field is updated along with the user preferences).

Issue(s)

Fix #23617

Steps to test or reproduce

1. Create a new direct message and send a message in another room (so that the DM is not shown on top of the list);
2. Go to My Account > Preferences > Notifications and update Desktop/Push/Email notification setting (any of them);
3. Return to the main screen and check the sidebar.
   Current behavior: empty DMs go to the top of the list.
   Expected behavior: empty DMs should now stay at the bottom of the list.

Further comments

The lastRoomUpdate variable is used as a backup field to the lm field when it is not available in the room (that is, when no message has been sent in the room). Changing user notification preferences also updates the _updatedAt field in every subscription, which implies that empty DMs will be sorted to top when the sidebar is sorted. Instead, we could keep this field as undefined so that they're sorted to the bottom of the list.

TC-79

[codecov]

Codecov Report

Merging #27912 (2c2fcdc) into develop (88f7024) will increase coverage by 1.48%.
The diff coverage is 54.36%.

@@             Coverage Diff             @@
##           develop   #27912      +/-   ##
===========================================
+ Coverage    42.24%   43.73%   +1.48%     
===========================================
  Files          812      790      -22     
  Lines        16052    15261     -791     
  Branches      2072     2085      +13     
===========================================
- Hits          6781     6674     -107     
+ Misses        8991     8308     -683     
+ Partials       280      279       -1     

Flag	Coverage Δ
e2e	43.73% <54.36%> (+1.48%)	⬆️

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