Move present fence await off main thread

The current path is, expectedly, causing jank.

Bug: 347376678
Test: record a trace of start flow, confirm wait is not on main thread
Flag: EXEMPT - bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:e9e12ca243cbe04bb03da9ab1ae161dfd966b381)
Merged-In: Ia05cb08855c324c0d2524a84905fcc696497816d
Change-Id: Ia05cb08855c324c0d2524a84905fcc696497816d