Skip epoll_wait syscall when possible

This change updates Looper to skip epoll_wait if there are no FDs
requested (see MessageQueue.addOnFileDescriptorEventListener()) and if
the poll timeout is 0, which indicates that there are already messages
to handle without another poll being needed.

Test: presubmit, atest libutils_test
Bug: 410218466
Flag: android.os.native_looper_skip_epoll_wait_for_zero_timeout

Change-Id: I4737fcf5b902576fa66e4886f99bccbaedb6e5fb