use android-9 for the android platform base rather than android-18

[froydnj]

Firefox compiles its Android port with version 9 of the Android platform (not sure if I have the terminology right here). The buildbot configs use version 18, but at least on x86 Android, compiling with version 9 works just fine, so I assume that's the case for the ARM Android port as well. If ARM Android works with version 9, I'd like to request that the automated builds use version 9 to make life more consistent for Firefox's sake.