Cap alpha to max if SAW + FLAG_NOT_TOUCHABLE

As part of go/untrusted-touches for S, we blocked touches from passing
windows of different UIDs regardless of targetSdk. For SAWs, we applied
an opacity rule where apps would be able to let touches pass through
(usually via FLAG_NOT_TOUCHABLE) as long as their cumulative opacity in
the touch area wasn't greater than 0.8.

This resulted in some overlay apps being broken, which we expected and
saw to some extent in df/dp/beta, but we had reached out and some apps
fixed the issue. But the issue seems to be more widespread and OEM has
escalated the issue, reporting it to be affecting them more severely
(see bug).

So, we automatically make SAW + FLAG_NOT_TOUCHABLE windows have 20%
transparency to try to honor FLAG_NOT_TOUCHABLE and let touches pass
through. Note that if the app has overlapping windows the cumulative
opacity would be taken into consideration and the touch could still be
blocked, but none among the apps I checked did that.

Test: atest -d android.server.wm.WindowUntrustedTouchTest
Test: Install and enable you.in.spark.energy.curved, verify touches work
      in the top part of the screen.
Bug: 218777508
Change-Id: I3a74e1cc43a36171ee10fbf68c4bfff538eee661