Add new Flag class. New FlagReaderPlugin.

This CL is a stepping stone towards a more complete flag system.

This cl allows for the definition of flags within systemui, each of
which have a unique id and a default value. Methods for retrieving
the value of a flag are defined.

By default, reading a flag always returns its default value.

A FlagReaderPlugin is also defined that, when loaded, allows for
returning alternate values for a given flag. The details of how an
alternate value are determined are left for the plugin to decide.

The plugin interface should be considered incomplete with this cl.
Methods for reading flags are defined, but hooks for knowing when
flags change (and hooks for knowing when a flag plugin has been
loaded), are not defined. Moreover, the flag plugin is not loaded
early in the process with this cl, so anyone who reads flags
before the plugin is may misss out on the values they care about.

More changes to come.

Bug: 194781951
Test: atest SystemUITests
Change-Id: I888d5eba82359a37878a3a4d4747990b79a77804