Fix File.getSizeOfFiles() double counting some files (#7907)

The `File.length()` docs state:

> Returns the length of the file denoted by this abstract pathname. The return value is unspecified if this pathname denotes a directory.

On Android, it *seems* to currently have the right behaviour, summing up the size of its children, which made `File.getSizeOfFiles()` double-count the size of some directories.

To avoid this, and be able to control the actual behaviour no matter what future Android versions do, let's count those ourselves.

libraries/androidutils/src/main/kotlin/io/element/android/libraries/androidutils/file/File.kt

@@ -84,5 +84,9 @@ fun File.getSizeOfFiles(): Long {
             Timber.v("Get size of ${it.absolutePath}")
             true
         }
-        .sumOf { it.length() }
+        // Only sum the actual size of files, not directories, to avoid double counting or worse,
+        // since the docs mention that `length()` for directories is not reliable.
+        // For directories, we can assume a size of 4096 bytes (4 KiB) as a placeholder,
+        // which is a common block size on many file systems.
+        .sumOf { if (it.isFile) it.length() else 4096L }
 }