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
Merged-In: I37ab41cf2ed7bc9cb6db6b4700c03ab48ed31fff
Change-Id: Ied45347c3e56e56d1372d51b98d7e52b02ff4755
(cherry-picked from commit 6e93f484)