Replace textPlaceholder color usages with textSecondary

[jmartinesp]

Content

Replaces usages of textPlaceholder with textSecondary.

Motivation and context

Improve accessiblity. cc @americanrefugee .

Screenshots / GIFs

In the PR. The placeholder texts should have more contrast now.

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/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly define what will change for the user
• 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

[github-actions]

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.87%. Comparing base (b772d4f) to head (c250ca2).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3873   +/-   ##
========================================
  Coverage    82.87%   82.87%           
========================================
  Files         1784     1784           
  Lines        45090    45090           
  Branches      5324     5324           
========================================
  Hits         37368    37368           
  Misses        5857     5857           
  Partials      1865     1865           

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