Enable missing dependency checks

With this enabled, we'll check the entire build graph for missing
dependencies -- even if you don't need to build the module. So it'll
present more errors, but surface them earlier.

These can be turned off manually with an environment variable if the
branch is expected to have missing dependencies:

  ALLOW_MISSING_DEPENDENCIES=true
  SOONG_ALLOW_MISSING_DEPENDENCIES=true   (previous method)

That will be set automatically when using 'mm', which doesn't load the
entire build graph. It will also be set when using 'tapas' to build
unbundled apps, since those are usually on reduced branches.

This provides better error messages when a library is missing:

system/core/init/Android.mk: error: init_tests (NATIVE_TESTS android-arm64) missing libinit (SHARED_LIBRARIES android-arm64)
Available variants:
  libinit (STATIC_LIBRARIES android-arm64)
  libinit (STATIC_LIBRARIES android-arm)
You can set ALLOW_MISSING_DEPENDENCIES=true in your environment if this is intentional, but that may defer real problems until later in the build.
system/core/init/Android.mk: error: init_tests (NATIVE_TESTS android-arm) missing libinit (SHARED_LIBRARIES android-arm)
Available variants:
  libinit (STATIC_LIBRARIES android-arm64)
  libinit (STATIC_LIBRARIES android-arm)
You can set ALLOW_MISSING_DEPENDENCIES=true in your environment if this is intentional, but that may defer real problems until later in the build.
build/core/main.mk:852: error: exiting from previous errors.

instead of:

ninja: error: 'out/target/product/generic_arm64/obj/SHARED_LIBRARIES/libinit_intermediates/export_includes', needed by 'out/target/product/generic_arm64/obj/NATIVE_TESTS/init_tests_intermediates/import_includes', missing and no known rule to make it

Test: Manually introduce errors, check for expected output
Test: multiproduct_kati on AOSP and internal master, no new breakages
Change-Id: I5847f813045929813214137403c499b44ef67907
Merged-In: I5847f813045929813214137403c499b44ef67907