Get rid of SYSTEM_ALERT_WINDOW permission in Android manifest

[gnprice]

Splitting from #3075:

• Test an install flow for any scary mentions of permissions. E.g., our manifest says we use the SYSTEM_ALERT_WINDOW permission.
  ...
• Remove the SYSTEM_ALERT_WINDOW permission from the output manifest in release builds; it's only needed in debug builds, for displaying the RN debug menu.

In fact, we've had logic to supposedly do exactly that since
2cc6a52 in 2017; with a fix in b5ac051 of 2018, and a refactor in a0f33ec of this week.

But a quick look at the actual manifest of 25.8.122, the latest beta, finds it's right there.

In fact... it looks like it was pulled in by React Native itself! Dates to the very beginning, reported as facebook/react-native#5886, and fixed as facebook/react-native#23504.

Which went into RN v0.59... which means it was fixed by the upgrade, #3399 . And, indeed, it's gone when I do a build now.

The issue was pretty confusing because RN redundantly had the permission in its own manifest fragment that gets merged into the output, and in the manifest in the app template. So anything we did to make it conditional in our own source manifest had no effect, and could only have had an effect if we'd used tools:remove -- because it never needed to be there at all, until just recently when it was removed (by that fix PR) from RN's own manifest fragment.