Fix ExecUtils.WaitChildWithTimeoutFallback by adding a fallback.

`WaitChildWithTimeout` didn't work with old Linux kernels because it
uses a syscall `pidfd_open`, which was added in Linux 5.4.

This change adds a fallback implementation of `WaitChildWithTimeout`
that creates a thread to wait instead of relying on `pidfd_open`.

Also:
- Use android::base::unique_fd to hold pidfd so that the fd is
  guaranteed to be closed.
- Allow timeout_sec to be negative, in which case it blocks until the
  subprocess exits.

Bug: 229268202
Test: m test-art-host-gtest-art_runtime_tests
Ignore-AOSP-First: Will cherry-pick later.
Change-Id: Iec3f21db135d9a8a3a915372253f1ff3e285e5cf