Make acksync optional for AOA initialization

Acksync is used to delay HID events until some request (in practice,
device clipboard synchronization) is acknowledged by the device.

This mechanism will not be necessary for OTG mode.

PR #2974 <https://github.com/Genymobile/scrcpy/pull/2974>

app/src/usb/aoa_hid.c

@@ -53,8 +53,6 @@ log_libusb_error(enum libusb_error errcode) {
 bool
 sc_aoa_init(struct sc_aoa *aoa, struct sc_usb *usb,
             struct sc_acksync *acksync) {
-    assert(acksync);
-
     cbuf_init(&aoa->queue);
 
     if (!sc_mutex_init(&aoa->mutex)) {
@@ -248,6 +246,11 @@ run_aoa_thread(void *data) {
 
         if (ack_to_wait != SC_SEQUENCE_INVALID) {
             LOGD("Waiting ack from server sequence=%" PRIu64_, ack_to_wait);
+
+            // If some events have ack_to_wait set, then sc_aoa must have been
+            // initialized with a non NULL acksync
+            assert(aoa->acksync);
+
             // Do not block the loop indefinitely if the ack never comes (it
             // should never happen)
             sc_tick deadline = sc_tick_now() + SC_TICK_FROM_MS(500);
@@ -294,7 +297,9 @@ sc_aoa_stop(struct sc_aoa *aoa) {
     sc_cond_signal(&aoa->event_cond);
     sc_mutex_unlock(&aoa->mutex);
 
-    sc_acksync_interrupt(aoa->acksync);
+    if (aoa->acksync) {
+        sc_acksync_interrupt(aoa->acksync);
+    }
 }
 
 void