force JavaVersion.VERSION_1_7 for all samples

[proppy]

With:

for f in */app/build.gradle; do sed -e $'s/^.*android.ndk/\
    compileOptions.with {\
        sourceCompatibility=JavaVersion.VERSION_1_7\
        targetCompatibility=JavaVersion.VERSION_1_7\
    }\
\
&/' $f; done

Closes #43

[tjohns]

I approve of this change 👍.

I wonder if we should do this with the rest of the framework samples...

[proppy]

@ggfan changed the SDL syntax as you suggested.

[ggfan]

LGTM, thanks!