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
Ignore-AOSP-First: security
Tag: #security
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:00c8b2a9cd94d0a2c71ccf4985b1789526310fce)
Merged-In: I69b2e80f0dd8682dcc8862b4b9aa7845c48c3bb8
Change-Id: I69b2e80f0dd8682dcc8862b4b9aa7845c48c3bb8