Dup first haptic channel to the second when it is not provided.

In this CL, if the device supports two haptic channels while the app
only provides one, the second haptic channel will be duplicating the
first haptic channel. With this change, it can help the users to get
benefit from two haptic channels when it is supported.

Bug: 203725447
Test: atest SoundPoolHapticTest
Test: play single and dual haptic channels
Test: audio smoke test
Change-Id: I6d54b65a5e48b9b49f5464523542aa4cc6a9efa3