Make activities in untrusted embedding mode invisible behind overlay

Activities embedded in untrusted mode must be made invisible when
there is another activity on top in the same task that belongs to
a different UID.

Also make sure the input mode is set to "drop when obscured" for
untrusted embedded activities.

Bug: 197364677
Test: TaskFragmentTrustedModeTest
Change-Id: I3f8e252fb2d8e5bffdc11b37c42cc41d116c8fa3