Add com.android.tethering apex to android.security flags

We want to reference one of the flags in the android.security package in
the Tethering apex (see aosp/3391690).

Flag: android.security.certificate_transparency_configuration
Bug: 379988507
Test: m
Merged-Id: I8a4664a4a668e19d849d714f3f34d49d278dc1ce
Change-Id: Idf8db889f7ec6226672f2b5ad6eb53b1a0a6760e