Fixed race condition that carrier config change event not notified

Fixed a race condition that when system unlocked happened while
the previous carrier config loading is still ongoing, the carrier
config changed callback is not sent. Fixed by adding a new flag
to check if normal carrier config loading process is ongoing.

Fix: 375526692
Test: atest CarrierConfigLoaderTest
Test: Basic telephony functionality tests
Flag: com.android.internal.telephony.flags.carrier_config_changed_callback_fix
Change-Id: I9292e0ba1d58c7d257a09e93e5c79018d65a43ca