Wait for carrier config to apply in tests

TetheringManagerTest#testRequestLatestEntitlementResult in particular
set carrier config overrides without waiting for the override to be
applied, which caused flakes.

Wait for the override to be applied when adding and clearing them.

This makes the logic to wait for the update in NetworkAgentTest
unnecessary.

Bug: 391966043
Test: atest
Change-Id: I371ff5742627ca0b1b6eb6be5f0fd19a0487ae83
2 files changed