Prefix ENABLE_MEDIADRM_64 with TARGET_

This build-time flag should be prefixed.

Bug: 37039525
Test: Built OS
Change-Id: I09ab0a1ed672a88db5741102d87973eb9267efb3
diff --git a/services/mediadrm/Android.mk b/services/mediadrm/Android.mk
index 1d5fa07..fa3a02b 100644
--- a/services/mediadrm/Android.mk
+++ b/services/mediadrm/Android.mk
@@ -43,8 +43,9 @@
 
 # TODO: Some legacy DRM plugins only support 32-bit. They need to be migrated to
 # 64-bit. (b/18948909) Once all of a device's legacy DRM plugins support 64-bit,
-# that device can turn on ENABLE_MEDIADRM_64 to build this service as 64-bit.
-ifneq ($(ENABLE_MEDIADRM_64), true)
+# that device can turn on TARGET_ENABLE_MEDIADRM_64 to build this service as
+# 64-bit.
+ifneq ($(TARGET_ENABLE_MEDIADRM_64), true)
 LOCAL_32_BIT_ONLY := true
 endif