init.rc: Restore the /mnt/sdcard symlink

"You are in a maze of twisty little symlinks, all alike."

Restore the /mnt/sdcard symlink, for compatibility with older Android
apps. This symlink was suppose to have been removed in the Gingerbread
time frame, but lives on.

Note: The /mnt/sdcard symlink was originally created in device specific
*.rc files in the device/vendor/hardware/* directory. This change moves
the creation of the symlink into the common init.rc file.

Bug: 25801877
Bug: 28108983
Change-Id: I2f9bf71bddffadb587d7376dfdfc8a546c84ec28