Add --dumpvar[s]-mode to replace config.mk uses

  build/soong/soong_ui.bash --dumpvars-mode \
    --vars="..." \
    --abs-vars="..." \
    --var-prefix="..." \
    --abs-var-prefix="..."

is similar to the previous:

  CALLED_FROM_SETUP=true BUILD_SYSTEM=build/core \
  make -f build/core/config.mk dump-many-vars \
    DUMP_MANY_VARS="..." \
    DUMP_MANY_ABS_VARS="..." \
    DUMP_VAR_PREFIX="..." \
    DUMP_ABS_VAR_PREFIX="..."

and

  build/soong/soong_ui.bash --dumpvar-mode [--abs] VAR

is similar to the previous:

  CALLED_FROM_SETUP=true BUILD_SYSTEM=build/core \
  make -f build/core/config.mk dumpvar-[abs-]-VAR

But uses soong_ui and ckati, so that we use a consistent make parser and
sandboxing configurations.

One major output difference between the pure make implementation and
this one is that report_config in Go is implemented using embedded
newlines in single quotes, while the make implementation uses `` with
embedded echo commands. This seems to work fine for both bash and zsh,
and report_config isn't meant to be machine-parsed anyways.

Test: build/soong/soong_ui.bash --dumpvar-mode report_config
Test: build/soong/soong_ui.bash --dumpvar-mode TARGET_DEVICE
Test: build/soong/soong_ui.bash --dumpvar-mode --abs PRODUCT_OUT
Test: build/soong/soong_ui.bash --dumpvar-mode --abs ALL_PRODUCTS
Test: build/soong/soong_ui.bash --dumpvars-mode --vars="report_config TARGET_DEVICE" --abs-vars="ALL_PRODUCTS"
Test: build/soong/soong_ui.bash --dumpvars-mode --vars=TARGET_DEVICE --abs-vars=PRODUCT_OUT --var-prefix=v_ --abs-var-prefix=a_
Change-Id: I0fbd0732bbf6fcfcd24084cf3c830a91a4b6bfc2