init: support wait timeout with more precision

A one second timeout is so coarse and can affect boot time when
the possibility that the file does not exist.  Switch to accepting
a floating point number for seconds for the wait for file command.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 151950334
Test: wait_for_file sleep 0.05 reports an appropriate delay
Change-Id: I8d8ed386519ab54270b05ce91663d0add30f12e7