Rename __ANDROID_SDK_VERSION__.

__ANDROID_SDK_VERSION__ is ambiguous. Rename it
__ANDROID_APEX_MIN_SDK_VERSION__ so it's clear which SDK version it
refers to.

Note that this is still different from the minSdkVersion of the module
being compiled. This is the max of *all* the minSdkVersions of modules
that this module shares an APEX with.

Test: treehugger
Bug: None
Change-Id: Id9cbd80a6bc99db8227daef4e1db9c893e63ad1e