Refactor power menu bug reporting flow to work for multiple admins

Existing implementation of power menu bugreporting flow works only
for the primary user. In HSUM(Headless system user mode) there is
going to be more than one admin users who are allowed to trigger bug
report from the power menu, below refactorings has been done do handle
this case.

Configurations for custom_bugreport_handler_app and custom_bugreport_handler_user are moved from Global to
Secure settings so that each user can save them.

The current context user will be the fallback default user in the
absence of a correctly configured handlerApp or handlerUser.

Retrieval of the current bug report handler apps are refactored to add
shell as user's default preferences for any user(current user).

Test: manually tested the bugreport is working for multiple admin with
different combination of handlerApp and handlerUser

Bug: 261184202
Change-Id: I3110c1f9619ede2edeb744daf37521318f0ff6e7