Fix ConcurrentModificationException when addTunable

Calls to addTunnable() in systemui are all in mainExecutor, we should
also use it in ScreenDecorations to prevent concurrent issue.

Bug: 199385132
Test: atest ScreenDecorationsTest
Change-Id: Idb8c3800c1bf222f57e212a89c6466367aa5d818
1 file changed