Add CAPTURE_BLACKOUT_CONTENT permission check for screenshots

Previously, only graphics and system were allowed to take screenshots of
layers that are normally blacked out. This change adds an additional check so
processes that have the permission CAPTURE_BLACKOUT_CONTENT can also take
screenshots of that content. Only system applications can have that
permission

Test: Builds, screenshots still work as before
Bug: 173746627
Change-Id: I53a1738be8e4787507a76f8b0ac703f3e413b30c