cleanup flash implementation

- fix issues with brightness control
- add separate classes for Android 5 and Android 6+ for simplicity
- properly cleanup camera in onDestory and also when stroboscope/flashlight mode are disabled

app/src/main/kotlin/com/simplemobiletools/flashlight/helpers/CameraFlash.kt

@@ -0,0 +1,12 @@
+package com.simplemobiletools.flashlight.helpers
+
+interface CameraFlash {
+    fun initialize()
+    fun toggleFlashlight(enable: Boolean)
+    fun changeTorchBrightness(level: Int) {}
+    fun getMaximumBrightnessLevel(): Int = DEFAULT_BRIGHTNESS_LEVEL
+    fun supportsBrightnessControl(): Boolean = false
+    fun getCurrentBrightnessLevel(): Int = DEFAULT_BRIGHTNESS_LEVEL
+    fun unregisterListeners(){}
+    fun release(){}
+}