getevent: O_RDWR -> O_RDONLY | O_CLOEXEC

No need to open the device read write. Just open it read-only.

This is a partial redo of
https://android-review.googlesource.com/c/platform/system/core/+/334583
which was reverted in
https://android-review.googlesource.com/c/platform/system/core/+/342483

Bug: 30861057
Test: ran getevent, saw correct output, played with device
Change-Id: I426efd68f39dcd97d3ee23c13ed1a9ed96cb4c2e