Split isEssentialPackage() from isSystemPackage()

Please use ApplicationInfo#isSignedWithPlatformKey() to check whether an
app is signed with platform key, instead of loading the signatures.

This is faster, and we also no longer need to load the PackageInfos with
the deprecated flag PackageManager.GET_SIGNATURES.

Bug: 235727273
Test: Unit test
Test: Manually with Settings App
Change-Id: I4d3c103559736b725517acef9bbca6a2c9d62ed6