Create sanitizer variants of APEX only when SANITIZE_TARGET is set

This fixes a problem that APEX is unconditionally mutated for all
sanitizer types. This can make an APEX to include sanitized version
of a lib even when SANITIZE_TARGET is not set. It can happen when lib X
is directly depended on by an APEX (e.g. via native_shared_libs) and X
has a sanitized variant due to the dependency from another library Y
which is force sanitized (via 'sanitize' property). In that case,
regardless of lib Y is in the APEX or not, the APEX chooses the
sanitized variant of lib X since the dependency from APEX to the lib is
created with AddFarVariationDependency.

Fixing this problem by mutating the APEX for a sanitizer type only when
the device is requested to be sanitized.

Bug: 122717287
Test: add libnetd_client to com.android.runtime APEX.
Inspect build.ninja to verify that libnetd_client.so in the runtime APEX
is not a sanitized one.

Change-Id: I918bc8407137d74c5456142b3a29de13df68c0b3