fix mediastore file size in SaveExternalFilesActivity

[sirambd]

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java.lang.IllegalArgumentException: column '_size' does not exist. Available columns: [_id, _data]
    at android.database.AbstractCursor.getColumnIndexOrThrow(AbstractCursor.java:351)
    at android.database.CursorWrapper.getColumnIndexOrThrow(CursorWrapper.java:91)
    at com.infomaniak.drive.utils.SyncUtils.getFileSize(SyncUtils.kt:55)
    at com.infomaniak.drive.ui.SaveExternalFilesActivity.store(SaveExternalFilesActivity.kt:407)
    at com.infomaniak.drive.ui.SaveExternalFilesActivity.storeFiles(SaveExternalFilesActivity.kt:352)
    at com.infomaniak.drive.ui.SaveExternalFilesActivity.access$storeFiles(SaveExternalFilesActivity.kt:63)
    at com.infomaniak.drive.ui.SaveExternalFilesActivity$setupSaveButton$1$1$1$1.invokeSuspend(SaveExternalFilesActivity.kt:247)
    at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
    at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
    at kotlinx.coroutines.internal.LimitedDispatcher.run(LimitedDispatcher.kt:42)
    at kotlinx.coroutines.scheduling.TaskImpl.run(Tasks.kt:95)
    at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:570)
    at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:750)
    at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:677)
    at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:664)

[sonarcloud]

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