CrOS Settings: Fix flaky OSSettingsDevicePagePower* test

This CL fixes flakiness of the related browser test. My suspicion is
the test failures were caused by a race condition of the order of
'battery-status-changed' events received. In order to predictably
control the order of these events, the tests are updated to wait for
the first event occurrence after element setup and before continuing to
the test execution.

Bug: b:303702729, b:305993270
Test: browser_test --gtest_filter="OSSettingsDevicePagePower*"
Change-Id: If29e4f6e2d4ce4c27523fadf056e654cc0733f71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4944892
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Wes Okuhara <wesokuhara@google.com>
Cr-Commit-Position: refs/heads/main@{#1211184}