hap: Improvements around the BluetoothHapPresetInfo builder

Since the BluetoothHapClient API pass BluetoothHapPresetInfo objects
only in one direction (when receiving BluetoothHapPresetInfo instances
from the service), the App shouldn't ever need to build them by itself.

From the Service point of view, preset index and name are both equally
important and are both mandatory thus should be both in Builder class
constructor, however in valid cases, preset info comes from the JNI
native code, wich does not use the builder class. In other rare cases
for invalid requests we should return null as API return params are
nullable. The builder can still be used in other parts like the unit
tests.

Bug: 150670922
Bug: 218682004
Bug: 220029662
Fixes: 218682004
Fixes: 220029662
Tag: #feature
Test: atest CtsBluetoothTestCases:android.bluetooth.cts.BluetoothHapClientTest BluetoothInstrumentationTests
Sponsor: jpawlowski@

Change-Id: I95c68406f531acc2056d3b16b3fcc8af4ca659cb