sm8450-common: Set board api level to 31

ims.apk checks ro.board.api_level and does not try the
aidl ims implementation if it's value is less than 31.

Change-Id: I524ee72785402a7f410d14a644ce2f43a56d0de3

common.mk

@@ -49,6 +49,7 @@ PRODUCT_PACKAGES += \
 
 # API
 PRODUCT_SHIPPING_API_LEVEL := 31
+BOARD_API_LEVEL := 31
 BOARD_SHIPPING_API_LEVEL := 31
 
 # Atrace