Split test utilities into host and device-only

Some test utilities can be used only by device-side tests because they
use the Android SDK, and some others are generic enough to be usable
both in host-side and device-side test.

Split the utilities into net-tests-utils-multivariant (compatible with
both host-side and device-side tests) and net-tests-utils (compatible
only with device-side tests) build targets.

Bug: 141836007
Test: atest FrameworksNetTests NetworkStackTests
Change-Id: Ic3944dffbaec22894631131602443097b4e83dd9