Fix soong_config handling of CompressedApex

add_json_bool treats non-empty variable as "true" by default (which is
conventional in make). Fix treatment of PRODUCT_COMPRESSED_APEX to
support the value false.

Bug: 185537646
Test: OVERRIDE_PRODUCT_COMPRESSED_APEX=false m nothing && \
      grep CompressedApex out/soong/soong.variables
Change-Id: I0562a375be377ab67a1d064e37aff5970c4ac8dd