Removes an already defined warning in aapt.

Apps that expect to use the current SDK don't specify anything in their
manifest and the build system rightfully injects the current SDK.

Any apps that specify their own <uses-sdk> or versionCode are telling
the build system that they are purposely using some other minSdkVersion,
etc (support library demos).

Test: None
Change-Id: I0aba3a71e15703c7bcafcf4d35608a1851dd8567
Fixes: 34901699