Use sdk_version: none for platform stubs

These stubs contain the libcore sources inside them, so need not
compile against core_current.

Also rename the defaults rule to be in line with the new neverallow.

Bug: 144149403
Test: m
Change-Id: I5f1b01875b0073f942c692882db157a8c5788ce0
Merged-In: I5f1b01875b0073f942c692882db157a8c5788ce0