Allow user to manually override enable_vabc_xor

--enable_vabc_xor=false to disable, --enable_vabc_xor=true or
--enable_vabc_xor to enable

Also the flag is now enabled by default. But update_engine will
still ignore XOR ops, as update_engine support for XOR is WIP.

Test: th
Change-Id: I038b0dda5cc9efcc1620e72c858626b56ffb1611

tools/releasetools/ota_from_target_files.py

@@ -221,6 +221,10 @@ A/B OTA specific options
       For VABC downgrades, we must finish merging before doing data wipe, and
       since data wipe is required for downgrading OTA, this might cause long
       wait time in recovery.
+
+  --enable_vabc_xor
+      Enable the VABC xor feature. Will reduce space requirements for OTA
+
 """
 
 from __future__ import print_function
@@ -286,7 +290,7 @@ OPTIONS.custom_images = {}
 OPTIONS.disable_vabc = False
 OPTIONS.spl_downgrade = False
 OPTIONS.vabc_downgrade = False
-OPTIONS.enable_vabc_xor = False
+OPTIONS.enable_vabc_xor = True
 
 POSTINSTALL_CONFIG = 'META/postinstall_config.txt'
 DYNAMIC_PARTITION_INFO = 'META/dynamic_partitions_info.txt'
@@ -1312,7 +1316,7 @@ def main(argv):
     elif o == "--vabc_downgrade":
       OPTIONS.vabc_downgrade = True
     elif o == "--enable_vabc_xor":
-      OPTIONS.enable_vabc_xor = True
+      OPTIONS.enable_vabc_xor = a.lower() != "false"
     else:
       return False
     return True
@@ -1357,7 +1361,7 @@ def main(argv):
                                  "disable_vabc",
                                  "spl_downgrade",
                                  "vabc_downgrade",
-                                 "enable_vabc_xor",
+                                 "enable_vabc_xor=",
                              ], extra_option_handler=option_handler)
 
   if len(args) != 2: