magisk: Prevent installation on Android 7.x and older

Android 7.x lacks the InMemoryDexClassLoader API, which is necessary for
the module to load Java code, and is unlikely to support hardware
attestation on any production devices anyway.

Fixes #124, #127

riru/template/magisk_module/customize.sh

@@ -33,6 +33,11 @@ else
   ui_print "- Device platform: $ARCH"
 fi
 
+# Check Android version
+if [[ "$(getprop ro.build.version.sdk)" -lt 26 ]]; then
+    abort "! This module only supports Android 8.0 and newer."
+fi
+
 # Extract libs
 ui_print "- Extracting module files"