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