base

Adds Settings.System.FONT_SCALE observer to ActivityManagerService

Changes to Settings.System.FONT_SCALE were not being handled by any service,
which required a device reboot for any changes to take effect.  Changes to
this field by the Settings app worked as expected only because it is able to
poke ActivityManager with an updated configuration, whereas unbundled
applications cannot.

This also ensures the setting value is backed up and doesn't conflict with
a configured value from accessibility onboarding during restore.

Bug:23033258
Change-Id: I98d4aed2f9f5893d054e6b10c4dfda406de8eba2