Add feature flag for disallow_cellular_null_ciphers

Adds a new aconfig file in android.os and creates
the first flag there.

Test: m
Bug: 276752881
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:5fc9dc6c0e9a121dd3556fd918088dbb14970af6)
Merged-In: I81b8f4f42bafc1adee139b35cc111dc852c4d2a2

Change-Id: I81b8f4f42bafc1adee139b35cc111dc852c4d2a2