Add flag android.service.notification.notification_silent_flag to RELEASE_CONFIG_TRUNKFOOD_STAGING.

This reverts commit 49fafdab31e9f527991e3d9b4949ac4c9fc5e274.

Reason for revert: b/470985267 -- this flag is in 'trunk_food', 'trunk', and 'bp2a' (next) but not 'trunk_staging'.  This is an error and needs to be fixed to make this flag consistent (and to ensure this flag is enabled in tests).

Change-Id: Iebb033f06f600d5d7b67e6ca0c0662f9a6126ec5