Reinstate functionality of IWifiChip.getAvailableModes()

IWifiChip.getAvailableModes() is needed as a dependency for VTS tests.
Reinstate the API to avoid breaking old tests, ignoring the new
AP_BRIDGED concurrency type.

Bug: 219617823
Test: atest VtsHalWifiV1_0TargetTest VtsHalWifiV1_1TargetTest
VtsHalWifiV1_2TargetTest VtsHalWifiV1_3TargetTest
VtsHalWifiV1_4TargetTest VtsHalWifiV1_5TargetTest

Change-Id: Ide5b251b20f1ea56a76c2ce3b04c4e28fc29624c