signapk: --align-file-size flag

A new flag tells that the resulting file size should be aligned as well.

For example, signapk -a 4096 --align-file-size ... generates the output
file of 4K-aligned sized.

Bug: 192991318
Test: check built APEX file size.
  APEX files should be sized as 4K-aligned.
Change-Id: I1c287e7219b4551bbb3e5957bdb64d80adfd3c39