Add TARGET_FORCE_APEX_SYMLINK_OPTIMIZATION

The APEX symlink optimization is a build-time trick to save the
storage/RAM usage of APEX by replacing some files in APEX with symlinks
to the files in the system partition. The optimization however is
automatically turned off for 'updatable: true' APEXes because doing the
optimization for them will hide the sys-health implication until when
the APEXes are built unbundled (i.e. prebuilt) and thus the
optimization is impossible.

TARGET_FORCE_APEX_SYMLINK_OPTIMIZATION forcibly disables the safety net.
When it is set to true, the symlink optimization is done regardless of
the 'updatable' property. This is useful for some of the devices like Go
where most APEXes (even the 'updatable: true' ones) should be
effectively non-updatable.

Bug: 175630508
Test: TARGET_FORCE_APEX_SYMLINK_OPTIMIZATION=true m and check that
updatable APEXes have symlinks to system libs

Change-Id: I26f72e5d5ebccc2d1e09c2a2f743db14937eb39a