Allow public-readable to persist.rcs.supported

For now, persist.rcs.supported has only vendor-init-settable, but it
turned out that the property should be read by vendor components in
some devices including 2018 Pixels.

Bug: 74266614
Test: succeeded building and tested on a blueline device with
PRODUCT_COMPATIBLE_PROPERTY_OVERRIDE=true

Change-Id: I926eb4316c178a39693300fe983176acfb9cabec