Set platform min sdk for Jack to o-b1

Test: m core-all
(cherry picked from commit 94acfe908b3e70cdd11920c92715ef06de7ed212)

Change-Id: I26a558677f2fa8ede09ad1ae7f31b786c4ba0a43
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index 91a3325..79ce53a 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -81,7 +81,7 @@
   # PLATFORM_SDK_VERSION. During development, this number may be incremented
   # before PLATFORM_SDK_VERSION if the plateform starts to add new java
   # language supports.
-  PLATFORM_JACK_MIN_SDK_VERSION := 25
+  PLATFORM_JACK_MIN_SDK_VERSION := o-b1
 endif
 
 ifeq "" "$(PLATFORM_VERSION_CODENAME)"