Nativehelper: Fix ScopedLocalRef

Follow-up to commit 076bf283.

Remove empty constructor, as it's dangerous to use. Remove null
assignment to mEnv as it's actually forbidden.

Runtime tests are introduced in a follow-up change.

Test: m
Test: manual
Change-Id: Ib8ad0e6b18eda57489b6ee4b325c50c360830fba