Improve `dumpsys backup`

This change refactors `dumpsys backup` code and adds some improvements -
	- Removes `isUserReadyForBackup()` check on calling user. This check is not needed, as it prevents a User with INTERACT_ACROSS_USERS_FULL permission, but not Backup activated, to get info about Backup service on other users. This is particularly relevant in case of headless mode, as Backup service isn't running for SYSTEM_USER.
	- Adds `--user <userId>` optional parameter for `dumpsys backup`, which will return info about Backup service for the user passed in argument. This is also helpul in case of multiple users on the device.

Bug: 267755698
Test: atest BackupManagerServiceTest
	Manual test by running dumpsys backup with optional arguments.
Change-Id: Id7790a501a8befb12a212f66adfc0cd8cc9f9e8b