bootstrap is added for cc_binary only when the module is in an APEX

Like shared libraries, the bootstrap subdirectory is appended to the
install path only when the module is in an APEX. Currently, this change
doesn't make any difference because only the Bionic binary (linker) is
satisfying the existing condition InstallToBootstrap() and it already
satisfies DirectlyInAnyApex.

However, this change makes a diference when we build the binary for the
host targets. Then without this change, it is installed to
out/soong/host/<OS>-<arch>/bin/bootstrap. This doesn't make sense
because we don't use APEXes for the host targets. With this change,
the binary is correctly installed to out/soong/host/<OS>-<arch>/bin
because DirectlyInAnyApex returns false for hosts.

Bug: 159685774
Test: m
Change-Id: I0d63bd8c11e3a96ee6a27b2295bc14d6dc15ff9e