closes #447
to test:
- click install on one app (app1), then press "back" button before download completes
- click install on another app (app2), then press "back" button before download completes
- wait for both apps to download and show 2 separate "tap to install" notifications
- tap the app2 notification
expected:
- app2 install dialog
result on previous code:
- app1 install dialog
- further click on app1 install dialog does nothing (since app download is no longer valid)
result on this code change:
- app2 install dialog comes up correctly
- further tap on app1 notification brings up app1 install dialog correctly