Check for existing discovery database in bta_hf_client_cb_init

Researcher finds an RCE related to HF client discovery database.

Changes against the related issue b/291281168 (guarded by flag below)
partially resolve this issue, but it is necessary to add a check in
another location.

Bug: 374746961
Bug: 356201480
Test: m libbluetooth
Test: researcher POC
Flag: com.android.bluetooth.flags.btsec_check_valid_discovery_database
Ignore-AOSP-First: security
Tag: #security
Change-Id: Ie6329743c8acdc72154b9e3774ca53e840c85478