Update trunk-stable flag for IPC test apis

Add a new flag to guard the IPC test mode APIs.  The old flag had been
deleted (through a revert) and re-added, which is not a supported process.
The old flag is now explicitly deleted.

The API file has been updated to reflect the new flag name.

NO_EXPORTED_FLAG_DELETION_CHECK="deleted flag never moved past DEV"

Flag: android.os.ipc_data_cache_testmode_apis
Bug: 396173886
Test: presbumit
Change-Id: If9c627802ab1ee2583a8b953973bab3bead61d4f