Allow sysUI to send a11y events for other package and user

As discussed, we:
* Allow apps with INTERACT_ACCROSS_USERS(_FULL) to specify an explicit
user (instead of just USER_CURRENT).
* Introduce new signature permission ACT_AS_PACKAGE_FOR_ACCESSIBILITY
and grant it to sysUI. This permissions allow holders to specify another
package on behalf of which they can perform a11y operations.

This is for toasts since now sysUI renders toasts on behalf of the app
for apps targeting R+.

Bug: 152839254
Test: atest FrameworksServicesTests:AccessibilitySecurityPolicyTest
      FrameworksServicesTests:AccessibilityWindowManagerTest
      android.widget.cts.ToastTest
Change-Id: I3541045d574518571f348051d53e24ff1a4a67ef