Add flush test

1. omx api allows flushing all ports at once by passing the
argument OMX_ALL. This fixture tests the flush call with the
arg OMX_ALL and expects behavior to be same as when the ports
are individually flushed.
2. Flush a port that is not listed and the api call shall
return with error code OMX_ErrorBadPortIndex
3. Test the behavior of flush API call when the component is
not in Execute State (expects OMX_ErrorIncorrectStateOperation)

Note: 2, 3 tests are disabled as of now

Test: make vts -j99 BUILD_GOOGLE_VTS=true TARGET_PRODUCT=aosp_arm64 \
&& vts-tradefed run commandAndExit vts \
--skip-all-system-status-check --primary-abi-only \
--skip-preconditions --module VtsHalMediaOmxV1_0Host \
-l INFO

Bug: 63796949

Change-Id: I6f0cccea0c923116f7e41ed0ef7d8d584631256b