Fix convertToModernFd to support /mnt/user paths

Since I72f39d3e35b975f0a386c055cbb10b4d21c21c86, the MediaProvider
now always opens FUSE paths when transcoding is required. Those paths
start with /mnt/user/ but apps can't access those paths directly so we
make the following changes:

1. Call readlink(2) directly (without stat as
ParcelFileDescriptor#getFile currently does).
2. Replace /mnt/user/<userid> with /storage paths and use the new
/storage path for scanning

This also fixes some failing CTS

Test: Apps don't ANR when calling MediaMetadataRetriever#setDataSource
Test: atest android.appsecurity.cts.ExternalStorageHostTest#testMediaNone29
Bug: 174655855
Fixes: 177860000

Change-Id: Iec48dcb714a47b147e16eae39df783de748960f4