Add an opaque OnceKey type and use it for all calls to Once in
build/soong. A future patch will convert the arguments to
Once* to OnceKey once users outside build/soong have been updated.
Test: onceper_test.go
Change-Id: Ifcb338e6e603e804e507203c9508d30ffb2df966
PDK builds need to use the latest SDK version instead of "current"
to match the behavior of Make.
Bug: 118634643
Test: sdk_test.go
Change-Id: Ice10d0ccb4066f27ce5839fc96a4026510057121
Split out SDK handling functions from java.go to sdk.go and tests
from java_test.go to sdk.go.
Test: sdk_test.go
Change-Id: I83ef48cbe5230572c1d4ecc0e89021d2f7c71b76