Fix getTempContainerId()

getTempContainerId() would always return "smdl2tmp1" unless you had
MAX_CONTAINERS number of SD card SDKs, because of an array sort that put
all the zeros at the beginning.

Switch from trying to find a hole in the series of numbers to just
getting a number that's one larger than the previous. This reduces the
algorithmic complexity and the memory requirements.

Bug: 2832580
Change-Id: I32dc75ef5a6645f594ea47b032d7402e8860ebcd