Abstract sdk_version string using sdkSpec type

The value format that sdk_version (and min_sdk_version, etc.) can have
has consistently evolved and is quite complicated. Furthermore, with the
Mainline module effort, we are expected to have more sdk_versions like
'module-app-current', 'module-lib-current', etc.

The goal of this change is to abstract the various sdk versions, which
are currently represented in string and is parsed in various places,
into a type called sdkSpec, so that adding new sdk veresions becomes
easier than before.

The sdk_version string is now parsed in only one place 'SdkSpecFrom', in
which it is converted into the sdkSpec struct. The struct type provides
several methods that again converts sdkSpec into context-specific
information such as the effective version number, etc.

Bug: 146757305
Bug: 147879031
Test: m
Change-Id: I252f3706544f00ea71c61c23460f07561dd28ab0