Ash: Migrate keyboard window test off RunUntilIdle

This changelist is part of an ongoing series to modernize Ash tests by
migrating them away from RunUntilIdle() and toward targeted waits for
the specific async signal or state transition each test depends on.

Replace the idle drain in the keyboard controller animation test with
the existing keyboard loaded-state helper. The test only needs the
mocked keyboard window to finish loading before resizing it.

Bug: b:506794349
Test: CQ
Change-Id: Iac2151d74954255e74c72fab2a2117763272b19b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7792521
Commit-Queue: Di Wu <diwux@google.com>
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1621708}