Add SOONG_SDK_SNAPSHOT_VERSION support

SOONG_SDK_SNAPSHOT_VERSION=current will generate unversioned and
versioned prebuilts and a versioned snapshot module. This is the
default behavior. The zip file containing the generated snapshot will
be <sdk name>-current.zip.

SOONG_SDK_SNAPSHOT_VERSION=unversioned will generate unversioned
prebuilts only and the zip file containing the generated snapshot will
be <sdk name>.zip.

SOONG_SDK_SNAPSHOT_VERSION=<number> will generate versioned prebuilts
and a versioned snapshot module only. The zip file containing the
generated snapshot will be <sdk name>-<number>.zip.

Bug: 157884619
Test: m nothing
      m SOONG_SDK_SNAPSHOT_VERSION=current art-module-sdk
      - check that the generated Android.bp file has not changed
        from the default.
      m SOONG_SDK_SNAPSHOT_VERSION=none art-module-sdk
      - check that the generated Android.bp file does not contain
        versioned modules.
      m SOONG_SDK_SNAPSHOT_VERSION=2 art-module-sdk
      - check that the generated Android.bp file only contains
        version 2 of each module.
Change-Id: I087e9d7d3ad110508a3d6a39bca50cbb46b3ce82