Revert "Revert "Allow empty IOmxStore implementation""

This reverts commit 77a5b891, which was
made originally for b/69110957.

Test: make vts -j123 && vts-tradefed run commandAndExit vts \
--skip-all-system-status-check \
--skip-preconditions -m VtsHalMediaOmxStoreV1_0Host \
-l INFO

Bug: 68491141
Change-Id: I4e0bd3c343811142995665672fe89ad3e9da6dde