Hide NO_THROTTLE_EXPIRY_TIME

There is no reason to expose NO_THROTTLE_EXPIRY_TIME since
the fact that the apn was not throttle can be explicitly checked
for by checking the throttle type.

Test: N/A
Bug: 175092152
Merged-In: I8888fae0037fd04e1668ab51eff91dd2bcb5e057
Change-Id: I8888fae0037fd04e1668ab51eff91dd2bcb5e057