Drop the dependency on AB_OTA_UPDATER flag.

This shortens the gap between A/B and non-A/B builds, by replacing the
dependency on build-time flag with runtime detection instead. It also
allows building and testing both paths regardless of the target OTA
type.

The size increase to /sbin/recovery looks negligible (< 0.01%).
 - marlin: increased from 2084928 to 2085024;
 - angler: increased from 2084776 to 2084896.

Test: Run recovery_component_test on angler and marlin.
Test: Sideload an A/B OTA package on marlin.
Test: Sideload a non-A/B OTA package on angler.
Change-Id: I1d927d1ede9713fb42f73b4fe324aa5705ee6f99

Android.mk

@@ -143,10 +143,6 @@ LOCAL_C_INCLUDES := \
 
 LOCAL_CFLAGS := $(recovery_common_cflags)
 
-ifeq ($(AB_OTA_UPDATER),true)
-    LOCAL_CFLAGS += -DAB_OTA_UPDATER=1
-endif
-
 LOCAL_MODULE := librecovery
 
 LOCAL_STATIC_LIBRARIES := \