Allow shell to revoke notification permission without kill

Add the revokePostNotificationPermissionWithoutKillForTest API, which
will allow the shell to revoke the POST_NOTIFICATIONS permission without
killing this app. Gate this permission behind the
REVOKE_POST_NOTIFICATIONS_WITHOUT_KILL permission, which is
signature|privileged, accessible only to the shell.

Ignore-AOSP-First: Contains information about unreleased features
Test: manual
Bug: 194833441
Change-Id: I3177d1aeb338591c1d736aa6b4f073b6db6227e7