Disable DBus dependent targets when "dbus" USE flag is not set.

Some targets should not be build when DBus is not enabled, such as the
update_engine_client, the generated DBus proxies and the DBus daemon.

This patch disables those headers and also temporary disables
libpayload_consumer and delta_generator on Android when the "dbus" USE
flag is not set, since they depend on headers that still depend on
DBus.

Bug: 25197634
Test: `mmma system/update_engine` on an android target without dbus.

Change-Id: Ic33dc52c2f910bd526b06663cdbd119a0b8cdfd9
diff --git a/update_engine.gyp b/update_engine.gyp
index 3fcb706..9b97e71 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -20,8 +20,10 @@
         'libbrillo-<(libbase_ver)',
         'libchrome-<(libbase_ver)',
       ],
-      # Defaults if the -DUSE_* flags are not passed to gyp is 0. You can set
-      # the default value for the USE flag in the ebuild.
+      # The -DUSE_* flags are passed from platform2.py. We use sane defaults
+      # here when these USE flags are not defined. You can set the default value
+      # for the USE flag in the ebuild.
+      'USE_dbus%': '1',
       'USE_hwid_override%': '0',
       'USE_mtd%': '0',
       'USE_power_management%': '0',