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/strings.xml

@@ -286,6 +286,8 @@
     <string name="overclock_enable_description">Higher values can make variable-framerate games run at a higher framerate, requiring a powerful device. Lower values make games run at a lower framerate, increasing emulation speed, but reducing the emulated console\'s performance.</string>
     <string name="overclock_title">Emulated CPU Clock Speed</string>
     <string name="overclock_title_description">Adjusts the emulated CPU\'s clock rate if \"Override Emulated CPU Clock Speed\" is enabled.</string>
+    <string name="synchronize_gpu_thread">Synchronize GPU Thread</string>
+    <string name="synchronize_gpu_thread_description">Synchronizing the GPU thread reduces the risk of games crashing or becoming unstable with dual core enabled, but can also reduce the performance gain of dual core. If unsure, select \"On Idle Skipping\". Selecting \"Never\" is risky and not recommended!</string>
 
     <!-- Log Configuration -->
     <string name="log_submenu">Log</string>