Don't use ctx.directlyInAnyApex for bootstrap libraries

The check for directlyInAnyApex is redundant for bootstrap libraries,
they are already explicitly enumerated in InstallToBootstrap, and
they are all directly in an apex.  Replace the check with
!ctx.isSdkVariant(), since ctx.directlyInAnyApex() was returning
false for the sdk variants.

Bug: 372543712
Test: no change to build.ninja
Test: all Soong tests pass
Change-Id: I00f63757bc93e50affcc54577393e34a5625154e