Fix delayed offload session start issue

This fixes the issue where there was a lag between the request to start
the offload session, and when actually it starts. We are fixing this by
acquiring the wakelock when the request comes in, and release it only
when power state transformation is confirmed.

Bug: 343987330
Flag: com.android.server.display.feature.flags.offload_doze_override_holds_wakelock
Test: Manual
Test: atest DisplayPowerControllerTest
Change-Id: I4de9b5ad844b613404cb66d9c2984fd8251c1343