Don't throw a runtime exception in getPackageInfo.

If the input is invalid, we need to just return null, otherwise we'll
make PackageManager crash.

Bug: 123676932
Fix: 123878869
Test: used a small app to query the API, reproduce the bug and
verified that the new code fixes it.

Change-Id: Icefb108cc95ec01e36021a5259c59dabdc24c4d6