[Export e2ee keys] use appName instead of element

[Claire1817]

Type of change

• Feature
• [] Bugfix
• Technical
• Other : Improvement

Content

Use the appName instead of the string "element" in the filename.

Motivation and context

In forked application like Tchap, we have "element" in the name of the file.

Screenshots / GIFs

For element debug

Tests

• Step 1
• Step 2
• Step ...

Tested devices

• Physical
• Emulator
• OS version(s): 30

Checklist

• 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
• Pull request includes a sign off
• You've made a self review of your PR
• If you have modified the screen flow, or added new screens to the application, you have updated the test UiAllScreensSanityTest.allScreensTest()

[github-actions]

Unit Test Results

  84 files  ±0    84 suites  ±0   1m 1s ⏱️ -8s
157 tests ±0  157 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
504 runs  ±0  504 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 4860ed9. ± Comparison against base commit a1f5493.

[github-actions]

Matrix SDK

Integration Tests Results:

• [org.matrix.android.sdk.session]
  passed="25" failures="17" errors="0" skipped="2"
• [org.matrix.android.sdk.account]
  passed="1" failures="1" errors="0" skipped="0"
• [org.matrix.android.sdk.internal]
  passed="1" failures="1" errors="0" skipped="0"
• [org.matrix.android.sdk.ordering]
  passed="1" failures="1" errors="0" skipped="0"
• [org.matrix.android.sdk.PermalinkParserTest]
  passed="1" failures="1" errors="0" skipped="0"

[bmarty]

LGTM, thanks!