__ANDROID_API__ tracks min_sdk_version

Previously, for cc_* modules, __ANDROID_API__ tracked the sdk_version
property. This however has caused a few number of problems:

1. It's confusing. __ANDROID_API__ has meant minSdkVersion. Therefore
the sdk_version property should mean minSdkVersion (since the macro
tracks the property). However, the introduction of the new
min_sdk_version property (which is currently for APEX) made this very
confusing. Also, this is not consistent with the java_* modules where
sdk_version means compileSdkVersion.

2. This is preventing go/android-future-symbols. The plan is to make the
APIs that are above the minSdkVersion available as weak symbols.
Previously those APIs had to be accessed via dlsym because they are
hidden behind the __ANDROID_API__ macro at build-time. To use make the
APIs visible at build-time, the module authors had to __ANDROID_API__
beyond their minSdkVersion. This is against the definition of
__ANDROID_API__.

To solve above problems, __ANDROID_API__ now correctly tracks
min_sdk_version. In addition, min_sdk_version now defaults to
sdk_version. Therefore, most of the modules that don't set
min_sdk_version aren't affected by this change.

Bug: 163288375
Test: m
Change-Id: I645e6bb1234c27ae0a69b7b87a59206cfd350744