Tidy up the definition of sdkCore.

Compared to the effect of the toModule function, this (a) sets
noFrameworkLibs to true, which seems logically correct, and (b) stops
setting java9Classpath, which was redundant since the same stubs are
available in 1.9+ builds via the systemModules setting. (This brings
sdkCore into line with sdkCorePlatform in both cases.)

Test: m art.module.api.annotations
Change-Id: I61a868bc363a3bf174725b78c0a1657021cfe08a