Install sdk variants in unbundled builds and package uninstallable variants

This effectively undoes both If6c3ee82d588e2742c85cef7244c090c93f38b8e
and I682e4f1f477f3024f7719dfaa67006ef335e0640.  SDK variants are now
installed again, which will fix unbundled builds of cc_test modules.
The platform variants used by com.android.virt are now packagable
even though they are not installable.

Fix the original problem in b/194403710 by adding a flag to platform
variants of modules in apexes that are not platform available, and
using that to prevent install and packaging dependencies.  That
allows the HideFromMake flag to go back to being used for preventing
install dependencies but not packaging dependencies.

Test: TestPackagingWithSkipInstallDeps
Test: TestFileSystemShouldInstallCoreVariantIfTargetBuildAppsIsSet
Test: TestFileSystemShouldSkipApexLibraries
Bug: 194403710
Bug: 268582372
Fixes: 274443025
Change-Id: If5418df3ddbb940bd631caebdf38daa81e71f40e