Make ab_ota_partitions read only

Now that we have moved the AB_OTA_BOOTLOADER_PARTITIONS &
AB_OTA_RADIO_PARTITIONS to BoardConfig, we can make the
ab_ota_partitions read only.

Eventually, the variable will be exposed as a product build
property. And we want its value to stay unchanged during the
build process.

Bug: 162148770
Test: build coral
Change-Id: I5258955a5c3303bdc61b97fc92f5dfa1905f7c37

core/board_config.mk

@@ -52,6 +52,7 @@ _board_strip_readonly_list := \
   TARGET_NO_RADIOIMAGE \
   TARGET_HARDWARE_3D \
   WITH_DEXPREOPT \
+  AB_OTA_PARTITIONS \
 
 # File system variables
 _board_strip_readonly_list += \