Security model for moving sharesheet to systemui

ResolverActivity (still in frameworks) now requests a "permission token"
that it hands to a stubbed system ui activity ChooserActivity.

This permission token allows an app (SysUI) with the signed permission
"START_ACTIVITY_AS_CALLER" to call
ActivityManagerService#startActivityAsCaller. Permission tokens are a
one-time use, limited-time offer.

Test: runtest systemui && manual testing
Bug: 69850752
Change-Id: I3600e1a8ff9eea7397f5f59853423c79b6401f98