Developer option for the app freezer

The app freezer allows the system to suspend execution for cached apps.
Without it an app can receive CPU time even if cached, so this option
will allow developers to test their code when frozen while their apps
are cached.

The new developer option will present a list of alternatives for the
freezer operating mode:

"device default": use device/system settings to determine whether the
                  freezer is to be enabled or not. This is the current
                  behavior, and default value for the option.
"enabled": force freezer on
"disabled": force freezer off

A dialog prompting the user to reboot is presented upon freezer mode
change. The device is automatically rebooted if the user decides to
proceed.

Bug: 155465196
Change-Id: I857b77cc4fc1ad766e8d5825ada9b271d3e1ab3a
Test: manually verifued the option works as intendend when this patch is
applied with its companion CLs

Change-Id: I26450b7b7ea2b345fd3661d4869fbc74ae38c0c9