[ADI][44/N] add @hide DEVELOPER_VERIFICATION_BYPASSED_REASON_UNSPECIFIED

(Addressing the comment in ag/34620716)

Adding the constant back but as @hide so that it is not exposed as an
API but can still be used as a placeholder or a default value in certain
places.

BUG: 360129657
FLAG: android.content.pm.verification_service
Test: atest android.content.pm.verify.developer.DeveloperVerificationSessionTest
Test: atest com.android.server.pm.verify.developer.DeveloperVerifierControllerTest
Change-Id: Ia33f913f9dc8d31e367e383b9aae92e1df6ed50f