Synchronize SplitController

Adding synchronization to the public methods in SplitController,
as they might be called on different threads and cause a
ConcurrentModificationException.

Bug: 228201646
Test: atest CtsWindowManagerJetpackTestCases --iterations 100
Change-Id: I2db9ccb5e9829487eae7aa85eff5f6153721cc04
1 file changed