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