Android: Add "Synchronize GPU Thread" setting (SyncOnSkipIdle/SyncGPU)

Many Android users want to disable SyncOnSkipIdle as a performance
hack, to the point where it's often suggested as something to
paste into Dolphin.ini (if not to use a fork). If adding it as
a setting in the GUI gives us an opportunity to explain what the
setting actually does and stops people from pasting stuff they
don't understand into INI files, I think it can be worth adding
despite how it can make games unstable. It not being in the GUI
doesn't seem to be stopping people from disabling it anyway.

The added setting in the GUI is a three-way setting called
"Synchronize GPU Thread" with the following alternatives:

"Never":            SyncGPU = False, SyncOnIdleSkip = False
"On Idle Skipping": SyncGPU = False, SyncOnIdleSkip = True
"Always":           SyncGPU = True,  SyncOnIdleSkip = True

Source/Android/app/src/main/res/values/arrays.xml

@@ -447,6 +447,17 @@
         <item>-1</item>
     </integer-array>
 
+    <string-array name="synchronizeGpuThreadEntries">
+        <item>Never</item>
+        <item>On Idle Skipping</item>
+        <item>Always</item>
+    </string-array>
+    <integer-array name="synchronizeGpuThreadValues">
+        <item>0</item>
+        <item>1</item>
+        <item>2</item>
+    </integer-array>
+
     <string-array name="motionControlsEntries">
         <item>Use Device Sensors (With Pointer Emulation)</item>
         <item>Use Device Sensors (Without Pointer Emulation)</item>