Use DEFAULT_APP_TARGET_SDK for dx and desugar

DEFAULT_APP_TARGET_SDK is PLATFORM_VERSION_CODENAME (for example 'P')
for non-release builds, and PLATFORM_SDK_VERSION (for example 25) for
release builds.  Use it for dx and desugar, and translate
PLATFORM_VERSION_CODENAME to 10000 to enable features in the current
development version.

Bug: 36087246
Bug: 36118520
Test: m -j ANDROID_COMPILE_WITH_JACK=false
Change-Id: I3d6bbc36a8869e4ced6bb850bc32f37b4cba3f85