set aria-selected on External Apps tabs
set aria-selected on External Apps tabs

test plan:
 * go to Course Settings / External Apps
 * ensure a screenreader tells you when a tab is selected
   (e.g., with VO, press Ctrl+Option+Space to select a tab, and
    VO reads "Not Installed, selected, tab")

fixes CNVS-35114

Change-Id: I16a4aa5c945ef47dad7f21467466827cce5b053b
Reviewed-on: https://gerrit.instructure.com/106073
Tested-by: Jenkins
Reviewed-by: Felix Milea-Ciobanu <9384aa226ac9d3237ed0bc52e18ffcb251ee0bf7@instructure.com>
QA-Review: Dan Sasaki
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

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