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"
(cherry picked from https://android-review.googlesource.com/q/commit:791d9aa87fa0a1503077a81fa80c51e4c6b620ee)
Merged-In: I77eb1596910ee1a6c9c2244beff7ff382ca57ba0
Change-Id: I77eb1596910ee1a6c9c2244beff7ff382ca57ba0