Dont show "Erase SD Card" when there is none

On devices with physcal, external SD Cards, the "Erase SD card"
option in in Settings/Storage is available even when there is
no SD card present, this dont make much sense since you cannot
erase a card that is absent. With this change the erase option
is bound to the mount toggle which leads to the following
consequences:

 Card present and mounted -> Erase enabled
 Card present but unmounted -> Erase enabled
 No card present -> Erase disabled
 Card present and MTP attached -> Erase disabled

Change-Id: I756ae8af3e0565febaf940df5f4991dd89105bd9