ApexManager: add API to get PackageInfo for given apexPackageName

This is a temporary API to mitigate a bug in StagingManager that uses
getActivePackage API incorrectly, i.e. instead of passing packageName
parsed from AndroidManifest.xml, it incorrectly passes packageName
from ApexInfo.aidl.

This is a quick-fix that can be CP-ed into beta4. Proper version will be
implemented in a follow-up CL.

Test: atest CtsStagedInstallHostTestCases
Test: on user build run:
      adb pull /system/apex/com.android.tzdata.apex /tmp/tzdata.apex &&
      adb install /tmp/tzdata.apex
      Verified install succeeded.
Bug: 132324953
Change-Id: I00ae711e93f32e81f53584d19c9ebc851757f205