Change the visibility of 'EOS_FLAG_BIT' or make it 'const' or 'readonly'

[nikcio]

Mentioned in #648

Specific issue: https://sonarcloud.io/project/issues?issues=AYRH0T92_qq9ReJdi431&open=AYRH0T92_qq9ReJdi431&id=apache_lucenenet

Make it const

Similar issues: https://sonarcloud.io/project/issues?resolved=false&rules=csharpsquid%3AS2223&id=apache_lucenenet

For internal members, try declaring private. But a compile will be required to test whether that is valid (some fields may actually need to be internal).

Public members should generally be made const if it is valid to do so, or static readonly in other cases. Again, compiling is required to test whether that works.

[paulirwin]

Note: Related to #662