Re-add mistakenly removed lock in SurfaceFlinger::removeLayer.

Lock was mistakenly removed with "Add detachChildren transaction"
when refactoring the weak-pointer to a strong-pointer.

Test: I will try and follow up with a stress-test in Transaction_test.
Bug: 36117490
Change-Id: I94171c86334bb52b974392186e1c3ebbf9f33a05
2 files changed