Move value for the Build.CUR_DEVELOPMENT constant into libcore

This matches a corresponding libcore change. It is to allow
libcore to use this value as a default when no other value is
explicitly set, e.g. in the system server or during testing
with vogar.

Bug: 34072201
Test: Device boots with CLs from this CL's topic.
Change-Id: Id812339a343ecea84a08bfdf4b873f3b156a13d4