OMS: fix offset-by-one error when setting overlay priority

setPriority(A, B) used to change A's priority to just higher than B's,
but in commit "OverlayManagerService: Make broadcasts/updates explicit"
this was modified so that A's priority is set to just lower than B's.

Revert the behaviour to the original since that is what is documented in
IOverlayManager.aidl

Test: manual
Change-Id: I87e96fb49d5ad58528dc0448aed907b405db1945