Fix AppOpsService setUidMode() when mode is 0 (MODE_ALLOWED).

Previously the code was calling SparseIntArray.get() without checking
for indexOfKey() >= 0 first, which leads to a default value of 0 that
collides with MODE_ALLOWED. This fix adds the check before get() is
called.

Test: manual
Change-Id: I04c183d6cd60cd1ec0babf47b905592c2fb62d4d