kmsutils: add function to synchronize with parent target state

Change-Id: I91765dd5091b66c0315cf2dfc440fe07eb843db3
kmswebrtcendpoint: use gst_element_sync_target_state_with_parent

Change-Id: Ie0e62f3f9ad9834ea19f14727f3b03098e9bdb87
kmswebrtcendpoint: improve syncing elements states with parent state

Now we do linking before sync state with the parent to avoid
race condition problems.

Change-Id: Ibdcbad4cb5bf224377746bf39a4e784985d8186c