Allow PLATFORM_BASE_OS override from product config

PLATFORM_BASE_OS is used to set value for
ro.build.version.base_os which is used to qualify
a device build as SMR (Security Maintenance Release)
during APFE (Android Partner Approval) to optimize the test span.
See ag/26859560 on how we plan to use this variable.

Add PRODUCT_BASE_OS that can be used to override
PLATFORM_BASE_OS value before making it read_only.

Plan is to set the value for PRODUCT_BASE_OS using build flags.
This way, it can be easily set per device when making on-demand
SMR releases. In future, move it to be set via soong variable
during build process once build system side is ready.

Test: None
Bug: 155105803

Change-Id: I7c4a1f291bb426ad599e1dc937e6ecd3889b5820