go/Android.bp: Clarify sdk_version documentation.

The documentation was unclear because there are at least three
different ways (codename, version, API level) to refer to each
version, see https://source.android.com/setup/start/build-numbers
It is further complicated by the fact that this value is taken
as a String even though the API level it refers to is normally
an integer. To disambiguate, this CL adds an example.

Test: Treehugger only.
Change-Id: I40c13104a87d16c84c2098f36f7f63447d3dda4d