add TARGET_BUILD_JAVA_SUPPORT_LEVEL to allow checking if java build is OK in PDK

- three levels: not defined, sdk, platform
- TARGET_BUILD_PDK_JAVA_PLATFORM gives the original meaning of java platform API
  supported for java.mk
- remove libexif from user tag
- libexif_jni is pulled from f/b/media

Bug: 6623618

Change-Id: I5fa085bd10f43963350c57b7f885c700de8963e5
diff --git a/core/java.mk b/core/java.mk
index 2b6851b..4e33ef2 100644
--- a/core/java.mk
+++ b/core/java.mk
@@ -4,7 +4,7 @@
 # all_res_assets
 
 ifeq ($(TARGET_BUILD_PDK),true)
-ifeq ($(TARGET_BUILD_PDK_JAVA),)
+ifeq ($(TARGET_BUILD_PDK_JAVA_PLATFORM),)
 # LOCAL_SDK not defined or set to current
 ifeq ($(filter-out current,$(LOCAL_SDK_VERSION)),)
 LOCAL_SDK_VERSION := $(PDK_BUILD_SDK_VERSION)