fix doing password stretching on main thread

[BillCarsonFr]

Type of change

• Bugfix

Content

DefaultSharedSecretStorageService was doing heavy computation on main thread.

Motivation and context

UX will freeze when creating secure backup (or reset)

• Changes has been tested on an Android device or Android emulator with API 21
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/vector-im/element-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-android/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes screenshots or videos if containing UI changes
• You've made a self review of your PR

[bmarty]

LGTM

[github-actions]

Unit Test Results

118 files  ±0  118 suites  ±0   1m 37s ⏱️ -4s
203 tests ±0  203 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
682 runs  ±0  682 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit a1e980d. ± Comparison against base commit cd52df5.