Fix small but crashy edge case in Icon.scaleDown

Plus a regression test which throws an Exception for the old version. It
shouldn't be an issue in the real world because there's absolutely no
reason any app should be creating 1920*3px icons.

Found while porting the code somewhere else.

Test: runtest -x ./core/tests/coretests/src/android/graphics/drawable/IconTest.java
Change-Id: I1283d982507221914ddad1313e16f63af13e245a