Add different explosion effect for Android.

It gives much better performance while it still looks similar.

data/gfx/explosion_low.xml

@@ -0,0 +1,30 @@
+<?xml version="1.0"?>
+<particles emitter="sphere" radius="0.1">
+
+    <spreading angle="180"         />
+    
+    <velocity  x="0.000"
+               y="0.0035"
+               z="0.000"           />
+    
+    <material  file="explode.png"  />
+    
+    <!-- Amount of particles emitted per second -->
+    <rate      min="100"
+               max="150" />
+    
+    <!-- Minimal and maximal lifetime of a particle, in milliseconds. -->
+    <lifetime  min="700"
+               max="900"         />
+    
+    <!-- Size of the particles -->
+    <size      min="0.8"
+               max="1.0"
+               x-increase-factor="0.6"
+               y-increase-factor="0.6"
+               />
+    
+    <color     min="255 255 255"
+               max="255 255 255"  />
+
+</particles>

src/graphics/explosion.cpp

@@ -42,8 +42,14 @@ Explosion::Explosion(const Vec3& coord, const char* explosion_sound, const char
     m_emission_frames = 0;
 
 #ifndef SERVER_ONLY
+    std::string filename = particle_file;
+    
+#ifdef ANDROID
+    filename = "explosion_low.xml";
+#endif
+    
     ParticleKindManager* pkm = ParticleKindManager::get();
-    ParticleKind* particles = pkm->getParticles(particle_file);
+    ParticleKind* particles = pkm->getParticles(filename);
     m_emitter = NULL;
     
     if (UserConfigParams::m_graphical_effects > 1)