Add PLATFORM_SDK_VERSION as a dexpreopt soong var

This is used to seed the corresponding --assume-value compiler
argument for SDK_INT compilation. Note that use of the arg is
still conditioned on the associated feature flag; when disabled,
the compiler argument is ignored.

Test: presubmit
Test: check dexpreopt compiler args in soong output
Test: check no change in dexpreopt w/ feature flag disabled
Test: m PrebuiltGmsCoreForClockworkWearable
Bug: 204924812
Change-Id: I662c7ac650ac12d1132f53b96c57373acde8ed52