Zipalign: Don't align directory entries

Directories are entries with uncompressed size zero and ending with
character '/' or '\' are allowed in apks since b/204425803. These
entries should not be considered for alignment since they are not
mmap by the framework.

Test: align_test.cpp
Bug: 250872480
Change-Id: I964aad118a82839f9ed230acc4c2c76f51888c67