OTA: Code cleanup

Change-Id: I8a82beaee2073977aa461ddad832a1548e5ed67d
OTA: Introduce otabsdiff

xdelta3 is currently having issues when executed in recovery. So,
Android's native apply_patch is used now. It is more accurate and more
sensitive to changes in system, which avoids errors when patching.

Change-Id: I49747b9e27551cc843861d72cc58e944a08eae08