Reserve space in super for dynamic partition metadata

The maximum size of all dynamic partition groups should not exceed
the super size - DAP metadata size. Today the configuration of
the device doesn't take the metadata into acount.

Since we recommend 4M to partners, reserve 4M here.

Bug: 183360400 
Bug: 182431975
Change-Id: Idd6d01db2594b4598c03aecbf931009fcf92a36c
Test: TH
diff --git a/pixel3_mainline/BoardConfig.mk b/pixel3_mainline/BoardConfig.mk
index 26fd66a..1ee5544 100644
--- a/pixel3_mainline/BoardConfig.mk
+++ b/pixel3_mainline/BoardConfig.mk
@@ -19,5 +19,5 @@
 BOARD_FLASH_BLOCK_SIZE := 131072
 # Super partition
 BOARD_SUPER_PARTITION_SIZE := 2952790016
-BOARD_DB_DYNAMIC_PARTITIONS_SIZE := 2952790016
+BOARD_DB_DYNAMIC_PARTITIONS_SIZE := 2948595712  # Reserve 4M for DAP metadata
 BOARD_SUPER_PARTITION_METADATA_DEVICE := system_b