Use Msf::Post::Android::System#get_build_prop to get the android ver

Instead of grabbing the android version from the module, this
is done by the mixin.

modules/post/android/manage/remove_lock.rb

@@ -4,11 +4,13 @@
 ##
 
 require 'msf/core'
+require 'msf/core/post/android'
 
 class Metasploit4 < Msf::Post
   Rank = NormalRanking
 
   include Msf::Post::Common
+  include Msf::Post::Android::System
 
   def initialize(info={})
     super( update_info( info, {
@@ -39,15 +41,25 @@ class Metasploit4 < Msf::Post
     ))
   end
 
-  def run
-    buildprop = cmd_exec('cat /system/build.prop')
+  def is_version_compat?
+    build_prop = get_build_prop
 
-    if buildprop.blank?
-      print_error("Blank build.prop, try again")
-      return
+    # Sometimes cmd_exec fails to cat build_prop, so the #get_build_prop method returns
+    # empty.
+    if build_prop.empty?
+      raise RuntimeError, "Failed to retrieve build.prop, you might need to try again."
     end
 
-    unless buildprop =~ /ro.build.version.release=4.[0|1|2|3]/
+    android_version = Gem::Version.new(build_prop['ro.build.version.release'])
+    if android_version <= Gem::Version.new('4.3') && android_version >= Gem::Version.new('4.0')
+      return true
+    end
+
+    false
+  end
+
+  def run
+    unless is_version_compat?
       print_error("This module is only compatible with Android versions 4.0 to 4.3")
       return
     end