Disable requestVisibleBehind.

As there is no caller for the SystemAPI convertToTranslucent, there is no situation
where requestVisibleBehind will actually result in the activity becoming
visible behind. However we have bugs in the requestVisibleBehind code-path,
so rather than fix them...it seems better to just prevent ourselves from
running in to them. Full deletion of the code-path is scheduled for post-O
branches.

Change-Id: I6e7c79e036986564d2d443a603e63c341de23057
Fixes: 62512584
Test: Repro from bug. go/wm-smoke.