SDK snapshot is dist'ed

`m module_sdk dist` produces snapshots of all SDKs in the source tree.
A snapshot is a zip file consists of Android.bp, exported headers,
exported AIDL files, stubs for native libs and jars. The zip file is
expected to be downloaded from the build server and extracted to a
directory (which probably will be
/prebuilts/module_sdks/<module_name>/current).

Bug: 138182343
Test: m (sdk_test.go updated)

Change-Id: Idbe4bc24795fe08f26fc1cf7497028f9d162053a