Fix thread booster

We need to conditionally set the prevPriority only if we enter
the first locked section. Otherwise we'll never reset back to the
lower priorities.

Test: Make sure no binder threads are stuck at 110.
Test: go/wm-smoke
Bug: 36631902
Change-Id: I8a9c329bc3084371022da91eabee45943c1b8c9f