Parse RELEASE_PLATFORM_BASE_SDK_EXTENSION_VERSION

Fall back to PLATFORM_SDK_EXTENSION_VERSION if value is not set

Bug: 375152642
Test: m sdk dist && unzip -p out/dist/android-sdk_linux-x86-*.zip android-sdk_linux-x86/samples/android-*/source.properties
Verify that isBaseSdk is still true when flag is undefined.

Change-Id: Id454176ef4146c9317a84161b5370b91e7e205e4