Set FakeFeatureFlagsImpl by field

Instead of calling the setter function, this change directly set the
FakeFeatureFlagsImpl in Flags by setting the field.

The change is required to make this rule useful for the test which uses
the production flag library. So the user don't need to duplicate all the
code.

Bug: 295561701
Test: atest FlagJUnitHostTests --host
Change-Id: If188a925389691497bb9922700e2484f3f8772af