Add ProgressBar to MediaDetailPagerFragment for Image Loading Indicator

[vtalos]

Description

This pull request introduces a ProgressBar to the MediaDetailPagerFragment to enhance the user experience by indicating when an image is loading. The ProgressBar is displayed while the image is being loaded and hidden once the loading is complete.

Fixes #5714

Changes Made:

1. Updated Layout File (fragment_media_detail_pager.xml):
   -Added a ProgressBar element to the XML layout file.
   -Positioned the ProgressBar appropriately within the layout to ensure it appears prominently when an image is loading.

2. Updated MediaDetailPagerFragment.java:
   -Integrated the ProgressBar into the fragment by finding the view and managing its visibility.
   -Implemented logic to display the ProgressBar while waiting for images to load and hide it once loading is complete.

Tests performed

Tested ProdDebug on Pixel 3 with API level 34

Screenshots

[nicolas-raoul]

Looks great!

Would you just mind fixing these related unit tests?

fr.free.nrw.commons.bookmarks.BookmarkListRootFragmentUnitTest > testRefreshNominatedMedia FAILED
    java.lang.NullPointerException at BookmarkListRootFragmentUnitTest.kt:261

fr.free.nrw.commons.bookmarks.BookmarkListRootFragmentUnitTest > testOnItemClick FAILED
    java.lang.NullPointerException at BookmarkListRootFragmentUnitTest.kt:292

fr.free.nrw.commons.category.CategoryDetailsActivityUnitTests > testOnMediaClicked FAILED
    java.lang.NullPointerException at CategoryDetailsActivityUnitTests.kt:69

fr.free.nrw.commons.contributions.ContributionsFragmentUnitTests > testShowDetail FAILED
    java.lang.NullPointerException at ContributionsFragmentUnitTests.kt:350

fr.free.nrw.commons.explore.ExploreListRootFragmentUnitTest > testRefreshNominatedMedia FAILED
    java.lang.NullPointerException at ExploreListRootFragmentUnitTest.kt:217

fr.free.nrw.commons.explore.ExploreListRootFragmentUnitTest > testOnMediaClicked FAILED
    java.lang.NullPointerException at ExploreListRootFragmentUnitTest.kt:167

fr.free.nrw.commons.explore.depictions.WikidataItemDetailsActivityUnitTests > testOnMediaClicked FAILED
    java.lang.NullPointerException at WikidataItemDetailsActivityUnitTests.kt:170
OpenJDK 64-Bit Server VM warning: Sharing is only supported for boot loader classes because bootstrap classpath has been appended

fr.free.nrw.commons.media.MediaDetailPagerFragmentUnitTests > testShowImage FAILED
    java.lang.NullPointerException at MediaDetailPagerFragmentUnitTests.kt:191

fr.free.nrw.commons.media.MediaDetailPagerFragmentUnitTests > testShowImageSingle FAILED
    java.lang.NullPointerException at MediaDetailPagerFragmentUnitTests.kt:197