Xperia S (LT26i) stolen. Can thief access internal storage? Recently my xperia s is stolen.I am confused what are the chances that thief can access and take my all videos and picture out from my phone.

I had App lock in my phone with admin right so no one can access settings or uninstall apps not even force stop.

if someone reset phone then only device memory format or internal storage too?

I had 4.1.2 Android (Jelly Bean).

Formatting and installing new android OS(With pc companion or Flashing) into my phone.Is device memory or both internal and device memory erased?
The device storage is in fact the internal storage, just mounted differently, so if you erase the device storage, you will also erase the internal storage. If the bootloader of the phone is already unlocked, he could flash a custom recovery like twrp and access the files from there (over adb for example). But if the bootloader is locked, in order to unlock it, you need to erase all user data (internal storage), so the thief would not be able to access any data.