Fix __ANDROID_SDK_VERSION__=0

__ANDROID_SDK_VERSION__ for a cc module means API level of which the
module should work with.

For APEX variants, it should be apex.min_sdk_version and tracked by the
variable cc.Module.apexSdkVersion.

The variable was set in the wrong place and used uninitialized
sometimes, which results __ANDROID_SDK_VERSION__=0.

Bug: n/a
Test: m
Test: checked build.ninja manually
      => no __ANDROID_SDK_VERSION__=0
Change-Id: Iba532b2c62773983414a061f5291a73363322487