Another attempt to workaround the glitch in unknown source test

The previous attempt break another test because FLAG_ACTIVITY_NEW_TASK
does not work with startActivityForResult.

And after a bit more testing, the issue (delay in showing the dialog)
happens when we start the activity in onCreate right away without waiting
for the completion of the rendering of itself (ByodHelperActivity).

By using the main thread handler, we show the dialog after
ByodHelperActivity is rendered, and the delay is no longer observed.

Tested the another relevant test "Enable non-market apps" is still working
properly with this workaround.

Fix: 63814740

Test: Run these two CtsVerifier tests:
1. BYOD -> Disable non market apps
2. BYOD -> Enable non-market apps

Change-Id: If03fd70dd969153a2680796676b278c551bee77f
1 file changed