Improve flash hidden element detection

[diracdeltas]

Separates plugin content setting from flashEnabled setting so that shields down
allows tiny flash elements to be detected. Also improves hidden element detection
for slow/delayed element loads.

Fix #4020

Auditors: @srirambv @bbondy

Test Plan:

1. go to reverso.net with flash enabled in about:preferences and shields down
2. enter something in the box and click translate
3. you should see a flash notification bar. click 'allow'
4. click on the copy-paste icon in the results box. the page should tell you that the text was copied.

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Test Plan:

[bsclifton]

Code changes look good; it'll try up to 3 times (1 second apart) to see if flash is on the page

Even though I felt like a horrible person installing Flash, I did install it and manually tested (code works great! 😄 )

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