libdm: Fix WaitForFile early-returning on failed accesses.

WaitForFile/WaitForDeletedFile both early return true if an error like
EPERM occurs. This was intentional because the code was modeled off
earlier fs_mgr code, but it makes libdm inherently racy if sepolicy is
not configured correctly. It's better to have these result in explicit
and consistent failures.

Bug: 148103327
Test: fastboot flashall
Change-Id: I0c78818962e1db91b556e523c418db28f7d78fae
Merged-In: I0c78818962e1db91b556e523c418db28f7d78fae