hap: Improve callback registration mechanism

Improves input parameters verification by throwing NPE as requested in
b/218616250. Adjusts the doc accordingly.

Bug: 150670922
Bug: 218616250
Tag: #feature
Test: atest CtsBluetoothTestCases:android.bluetooth.cts.BluetoothHapClientTest
Sponsor: jpawlowski@
Change-Id: I828d578ca53678a2642bc4e73d57b7b481feb88b