Fix flaky RefreshRateStatsTests

* Replace ASSERT_* with EXPECT_* so that tests no longer fail fast when
an expectation is not met.
* Remove initial assertion for screen off times, as metrics can tick
before the initial stats flush.
* Store relevant prior metrics for assertions after config/power mode
changes.

Bug: 124318277
Test: atest libsurfaceflinger_unittest --serial 92JBA03174 --
--module-arg
libsurfaceflinger_unittest:native-test-flag:--gtest_repeat=10000
--module-arg
libsurfaceflinger_unittest:native-test-flag:--gtest_filter=RefreshRateStatsTest*
Change-Id: I4670b5f86667977484d55980252ae765b5108efe