Use ApplicationInfo.isSignedWithPlatformKey()

To check whether an app is signed with platform key, instead of loading
the system signature.

And we also no longer need to load the PackageInfos with the deprecated
flag PackageManager.GET_SIGNATURES.

Bug: 235727273
Test: Manually with Settings App
Change-Id: I8af59268c48d50dd0c98fa6fc98b1c82727762cc