Remove `oneway` from onHotplugEvent callback

IIUC, oneway makes the callback a non-blocking call. Hwcomposer
implementations are expected to call onHotplug/onHotplugEvent during
the initial hwcomposer->registerCallback(). However, a oneway callback
would not block and cause the hwcomposer's implementation of
registerCallback() to return immediately. Example log:

11-30 23:32:32.962   508   508 E RanchuHwc: registerCallback
11-30 23:32:32.962   508   508 E RanchuHwc: registerCallback:
display:0
11-30 23:32:32.962   508   508 E RanchuHwc: setCallbacks for display:0
11-30 23:32:32.962   508   508 E RanchuHwc: registerCallback:
display:1
11-30 23:32:32.962   508   508 E RanchuHwc: setCallbacks for display:1
11-30 23:32:32.962   508   508 E RanchuHwc: registerCallback initial
registration, hotplug connecting display:0
11-30 23:32:32.962   508   508 E RanchuHwc: registerCallback initial
registration, hotplug connecting display:1
11-30 23:32:32.962   572   592 E SurfaceFlinger: jasonjason
SurfaceFlinger::onComposerHalHotplugEvent()
11-30 23:32:32.962   572   572 F SurfaceFlinger: Initial display
configuration failed: HWC did not hotplug
11-30 23:32:32.962   572   592 E SurfaceFlinger: jasonjason SurfaceFlinger::onComposerHalHotplugEvent()
<SurfaceFlinger aborts from the Fatal "HWC did not hotplug">

Bug: b/314208907
Test: Launch Cuttlefish with ag/25521493 then
      vts -m VtsHalGraphicsComposer3_TargetTest
Change-Id: Ibfea0b21e1d4c8b0de17bbe25dc55483050700b7