Add environment variable UNBUNDLED_BUILD_TARGET_SDK_WITH_DESSERT_SHA

UNBUNDLED_BUILD_TARGET_SDK_WITH_DESSERT_SHA enables user to specify
${codename}.${sha} as min_sdk_version and target_sdk_version in order to
support mainline train building into android build.

Test: TH
Bug: 295905124
Change-Id: I032d464aaa7314520f98b8eafa53d962940075ce