Allow ROLE_HOME to use FLAG_SLIPPERY

Some launchers don't have the "signature" permission.
Instead of requiring "signature | recents" protection level for
FLAG_SLIPPERY, let's change this to provide such permission to any
"home" app. This way, any launcher can use this flag.

This CL can be verified by pulling down the notification shade from home
screen. The pull must happen from the middle of the screen, not from the
top of the screen.

Bug: 220805921
Bug: 157929241

Test: atest PermissionControllerMockingTests
Test: verified on a pixel device
Test: atest android.permission2.cts.PermissionPolicyTest
Change-Id: I83a5c45ba818374c823065c8d104c06bed3e1459