Make signapk align .so entries to 4096 bytes.

This makes signapk align uncompressed .so entries to memory page
boundary (4096 bytes) to enable such libraries to be loaded at runtime
through memory-mapping the APK.

With this change in place, there should no longer be a need to run
zipalign after (or before) signapk.

Bug: 25794543
Change-Id: I74775af15a683791f57fcbd3497a79951b3f63a1