Update external storage permission strings

Re-phrase the strings not to refer to the specific type of storage
the particular device has (e.g. "sd card" or "usb storage"). The way
this was previously achieved was by compiling the resources differently
depending on the device. Eliminate these strings to make the framework
resources generic.

Note that these strings describe a permission that is in the process
of being deprecated.

Bug: 118618261
Test: inspect descriptions on blueline
Change-Id: I50bb7cae67bc39138bbe88f04b75c9ac750979cb