Adjust native API level properly when version L is available

Since ANDROID_SUPPORTED_NATIVE_API_LEVELS is not sorted
and might contain L instead of only number, adjusting the level
fails as it compares L > the version number. L being a character,
its value is 0 and the test passes while it should not.

For example SET( ANDROID_SUPPORTED_NATIVE_API_LEVELS 8 L 3 )
and ANDROID_NATIVE_API_LEVEL is 8
then the adjusted native API level drops to 3
while it should remain 8.

Fix : remove characters from ANDROID_SUPPORTED_NATIVE_API_LEVELS