Fixed DisplayCutout flaky test

The root cause is that only the top cutout spec was cached.

Now cache the full spec string.

Bug: 145983914
Test: atest android.view.DisplayCutoutTest#fromSpec_setsSafeInsets_top_and_bottom, android.view.DisplayCutoutTest#fromSpec_setsSafeInsets_top -- --abi x86
Test: atest DisplayCutouTest, ScreenDecorationTest
Change-Id: I21ef26e5453412ec659b1d05798b61f04c3d660b
Merged-In: I1d5663ebd8588021387b7ae5296851769954f996