Fix InputMonitor.updateInputWindowsLw

Drag and drop (as well as window positioning by drag)
was broken by commit 6213caa4. Drag-related window
handles got wiped out by a call to clearInputWindowHandlesLw.

Bug: 33499942
Test: cts-tradefed ... android.server.cts.CrossAppDragAndDropTests
Change-Id: I2504b1b6623fc8551c8110606c06156e49bfc63d