[aapt2] fix the comments from older review

- add a test case for the null last entry in sparse types
- add comments explaining what mLength is for sparse types
- remember if the type is sparse in a member
- pragma once

Test: atest libandroidfw_tests
Flag: EXEMPT bugfix / refactor
Change-Id: I7c2e516e8887f5fd1c2ab35b20fb49864b6761b6