Revise the checking logic of checkPackage

This method used to do a quick check to validate if a package name
belongs to a UID. However, it also returns positive when the given
package isn't existed.

Test: atest AppOpsTests CtsAppOpsTestCases AppEnumerationTests
Bug: 203430648
Change-Id: I5c4702df73829178775862cce8fbd43f2c3a3d8f