Build platform with core library stubs by default

Switch targets that don't explicitly specify an sdk_version
to using core.platform.api.stubs instead of the core library
implementations.

Note: the old core-oj target contained the source for
core-lambda-stubs too. The new core.platform.api.stubs
does not contain core-lambda-stubs so it is explicitly
added to the boot classpath list as part of the switch over.

Bug: 113148576
Test: build
Change-Id: I689d2127f6b69b4a7034fb96f3d9cf714f801794