Enable Async merge during OTA

If the async merge has a temporary failure, gracefully fallback
to synchronous I/O without failing the OTA merge.

Bug: 220991038
Test: 1: OTA on CF pre-submit runs
2: OTA on O6, R4 and Bramble with the following flow on TP builds:

BUILD-1 (without-this-patch) -> BUILD-2 (with this patch) -> BUILD-3 (with this patch)

Verified the failure path of async merge on all three devices

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I0b499d2cc81f44c74f85745f9165b81d0ce34eba