A new Client-Side FeatureFlags library.

This is just a stub at the moment. Follow-on cls contain more
implementation.

This cl describes the basic shape for usage:

  BooleanFlag FOOBAR = FeatureFlags.booleanFlag("foo", "bar", false);
  FeatureFlags.getInstance().isEnabled(FOOBAR);

Defined are 4 flags types:
  - BooleanFlag: can be either true or false. Held constant until
        device restart.
  - DynamicBooleanFlag: can be either true or false. Can change while
        a process is running.
  - FusedOnFlag: Always true.
  - FusedOffFlag: Always false.

More flag types are intended (Resource & SystemProperty backed,
Strings, Floats, Ints, etc). but this cl represents the MVP.

More about the details of this client can be read at:
http://go/android-flagging-dd

Test: m Android-Flags
Bug: 279054964
Change-Id: I0e455bf69f01dc675ec8bf64e584e2beff4e72ce