Add support for *_CPU_VARIANT_RUNTIME flags

Specified in the BoardConfig, and default to the value of the non
_RUNTIME variants.

These flags do not modify build-time compilation, but get written to
system properties on /vendor and can be inspected at runtime to
run/compile code for the specified cpu.

Migrate the bionic and ART cpu variant flags to use the new variables,
but leave the ART preopting using the non-_RUNTIME variants.

Bug: 120773446
Test: boot downstream device specifying CPU_VARIANT_RUNTIME flags
Test: grep dalvik.vm vendor/default.prop
Change-Id: Idc81110172582f38f84cca87f2eff140c5b5f9ac