Single out Register events LE_META_EVENT as special

LE_META_EVENT is a special event that multiplexes LE
events underneath and should not be exposed.

Test Parameters: INIT_gd_hci=true

Bug: 166280067
Tag: #refactor
Test: gd/cert/run --host
Test: ble paired 2 phones
Test: act.py -tc BleCocTest
Change-Id: Idf8dd9fa9e13fc9e2d1d7d89d6af0a16ec574b89