Don't treat pthread_t as a signed value type

1. HID host treats pthread_t as a signed value. Some platform may define
it as unsigned type.
2. HID host uses -1 as invalid pthread ID. pthread does not define
invalid thread ID.


Bug: 285750089
Test: Floss test "Two BLE connections"
Change-Id: I77eb1596910ee1a6c9c2244beff7ff382ca57ba0