Allow requesting start-activity-permission-token for an activity

The permission token was required to be requested from an existing
activity. Since the token is delegated to another activity, the
activity that requested the token had to keep alive and finish itself
until the token is revoked.

Changing the behavior to allow the caller to request the permission
token for a particular component as long as the caller is running in
System uid (which not necessarily to be an Activity).

This also prevents having an additional trampoline activity launched.

Bug: 199743918
Test: locally tested
Change-Id: If3e55032bb99d98a446e4c30220555b0fbc0c6f7