record read only flags in test helper class

This change adds a public method isFlagReadOnly in CustomFeatureFlags
class. The testing tools can call this method to decide whether a given
flag is read only or read write.

This change also make the related fields and methods to static.

Test: atest aconfig.test
Bug: 412743688
Change-Id: If91fa79c0a6a5e58a37eae34b5f86429dc31fbaa