installd: Wait indefinitely for the subprocess if pidfd_open fails.

In Android T we introduced a feature in installd, which is to kill the
subprocess that times out. This feature is implemented by a syscall
pidfd_open, which requires kernel 5.4 or above, so it fails on older
kernels, and therefore causes installd unable to invoke dex2oat, etc.,
making the device unable to do any app compilation.

After this change, if pidfd_open fails, installd will wait indefinitely
for the subprocess, which is the behavior in Android S.

Bug: 238316970
Test: Presubmit
Change-Id: I12e492937b876b5b312b3f8eedce6e5d5d2a4a4e