Add SIGNAL_REBOOT_READINESS permission

Adds a new permission (privileged or signature) which will
allow the holder to register callbacks with the
RebootReadinessManager. Also grants this permission to the
shell for testing purposes.

Test: atest CtsRebootReadinessTestCases
Bug: 161353402
Change-Id: Ide2ec7a0c2b905b4986031f2b6448182b45bb07a