Pandora: Fix service discovery for HAP tests

Clear gatt cache to make sure discovery takes place when needed so
PTS detects it properly.

Bug: 394789648
Test: atest pts-bot:HAP/HARC/CGGIT/CHA/BV-04-C -v
Flag: EXEMPT, test only
Change-Id: I878cd46ab98bbd8ed62982b13d33195d1b8a5134