Fix InsetsState#set if this == other

The logic shouldn't clear or remove the sources if this and the given
InsetsState are the same object. Otherwise, we will lose the reference
to the source that needs to be set from.

Fix: 417997812
Flag: EXEMPT bugfix
Test: atest InsetsStateTest
Change-Id: Ib8daec735f048731719353d3c52cb322fd70b964