Add content description to FAB

[zwarm]

Parent #15077

This PR fixes lint error by adding content description to:
media_picker_fragment.xml
photo_picker_fragment.xml

For more details see: paqN3M-mc

To test:

• Open your device's Settings app.
• Navigate to Accessibility and select TalkBack.
• At the top of the TalkBack screen, press On/Off to turn on TalkBack.
• In the confirmation dialog, select OK to confirm permissions.
• Navigate to stories and tap the FAB
• Listen for the content description 'Open Camera'
• Navigate to media picker and tap the FAB
• Listen for the content description 'Open Camera'

Regression Notes

1. Potential unintended areas of impact 🟢
2. What I did to test those areas of impact (or what existing automated tests I relied on) 🟢
3. What automated tests I added (or what prevented me from doing so) 🟢

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APKs:

• Jetpack-pr-15085-build-113117.apk
• WordPress-pr-15085-build-113117.apk

[ParaskP7]

👋 @zwarm !

I have reviewed and tested this PR as per the instructions, everything works as expected, good job! 🌟