Add sdk extension version info to prop templates

This adds two new properties alongside AndroidVersion.ApiLevel:
- AndroidVersion.ExtensionLevel: The sdk extension version if the SDK
- AndroidVersion.IsBaseSdk: Whether the extension level is the the one
  that the ApiLevel shipped with (true), or if is an extension SDK
  release (false).

While we're here, dedupe the recipe that processes these templates.

Bug: 195281582
Test: m out/host/linux-x86/development/sdk/platform_source.properties
      and inspect the substitution is working
Change-Id: Ieea20731aed2e0f35bbe4cfd8ca596cf11f4b3bc
Merged-In: Ieea20731aed2e0f35bbe4cfd8ca596cf11f4b3bc
(cherry picked from commit cf2f42c36b7997e9ce87682bb3a5511bdb72936f)

sdk/doc_source.prop_template

@@ -2,3 +2,5 @@ Pkg.UserSrc=false
 Pkg.Revision=1
 AndroidVersion.ApiLevel=${PLATFORM_SDK_VERSION}
 AndroidVersion.CodeName=${PLATFORM_VERSION_CODENAME}
+AndroidVersion.ExtensionLevel=${PLATFORM_SDK_EXTENSION_VERSION}
+AndroidVersion.IsBaseSdk=${PLATFORM_IS_BASE_SDK}