Create scripts to update and freeze a module SDK

`m <sdk_name>` generates two scripts each of which is use to update the
current snapshot of the SDK and to freeze ToT as a new version,
respectively. Executing the scripts will copy necessary files (stub
libraries, AIDL files, etc.) along with Android.bp into the ./<apiver>
directory under the directory where the sdk is defined.

This change also introduces a new module type 'sdk_snapshot' that
represents a snapshot of an SDK. It will be auto-generated by the above
scripts, so developers are not expected to write this manually.

The module type 'sdk' is now used to simply specify the list of modules
that an SDK has.

Finally, this change changes the version separator from '#' to '@'
because '#' confuses Make.

Bug: 138182343
Test: m

Change-Id: Ifcbc3a39a2f6ad5b4f4b200ba55a1ce3281498cf