Fix the progress getting for mapped files

Mapped files don't support querying their loading progress,
so we should simply skip them - they are already a part of
some other file, and will get accounted for loading when
that file's progress get queried

+ a bunch of small improvements

Bug: 180535478
Test: atest service.incremental_test, adb install --incremental
  with mapped native libs

Change-Id: Ifc8a402144f2f3669a0419124fb0f35d7002190a