Merge "Mark BUILD_NUMBER as obsolete, more vars as readonly"

# Build System Changes for Android.mk Writers

## `BUILD_NUMBER` removal from Android.mk  {#BUILD_NUMBER}

`BUILD_NUMBER` should not be used directly in Android.mk files, as it would

trigger them to be re-read every time the `BUILD_NUMBER` changes (which it does

on every build server build). If possible, just remove the use so that your

builds are more reproducible. If you do need it, use `BUILD_NUMBER_FROM_FILE`:

``` make

$(LOCAL_BUILT_MODULE):

	mytool --build_number $(BUILD_NUMBER_FROM_FILE) -o $@

```

That will expand out to a subshell that will read the current `BUILD_NUMBER`

whenever it's run. It will not re-run your command if the build number has

changed, so incremental builds will have the build number from the last time

the particular output was rebuilt.

## `DIST_DIR`, `dist_goal`, and `dist-for-goals`  {#dist}

`DIST_DIR` and `dist_goal` are no longer available when reading Android.mk

$(shell mkdir -p $(OUT_DIR) && \

    echo -n $(BUILD_NUMBER) > $(OUT_DIR)/build_number.txt)

BUILD_NUMBER_FILE := $(OUT_DIR)/build_number.txt

.KATI_READONLY := BUILD_NUMBER_FILE

$(KATI_obsolete_var BUILD_NUMBER,See https://android.googlesource.com/platform/build/+/master/Changes.md#BUILD_NUMBER)

ifeq ($(HOST_OS),darwin)

DATE_FROM_FILE := date -r $(BUILD_DATETIME_FROM_FILE)

else

DATE_FROM_FILE := date -d @$(BUILD_DATETIME_FROM_FILE)

endif

.KATI_READONLY := DATE_FROM_FILE

# Pick a reasonable string to use to identify files.

ifeq ($(strip $(HAS_BUILD_NUMBER)),false)

else

  FILE_NAME_TAG := $(file <$(BUILD_NUMBER_FILE))

endif

.KATI_READONLY := FILE_NAME_TAG

# Make an empty directory, which can be used to make empty jars

EMPTY_DIRECTORY := $(OUT_DIR)/empty