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 handlerUser and handlerApp are moved from Global to
Secure settings so that each user can save them, and current user's
context is used to save and retrieve these secure settings.

In case of bugreports triggered from power menu the caller is always
going to be systemui running on user 0, ActivityManagerService has
been refactored to always initiate the shell process for bug reporting
on the current user.

Test: manually tested the bugreport is working for multiple admin with
different combination of handlerApps

Bug: 261184202
Change-Id: I5ea8e3b8b7cf9d032bfa3f3065aee5eb4f3da068