Don't instantiate unused power buttons in StatusBarService.

New power buttons in the StatusBar is a great thing. But, on my HTC Hero,
I got a lot of the following error in logcat:

E/FlashlightButton(  133): getFlashlightEnabled failed
E/FlashlightButton(  133): java.io.FileNotFoundException: /sys/class/leds/flashlight/brightness (No such file or directory)
E/FlashlightButton(  133):      at org.apache.harmony.luni.platform.OSFileSystem.openImpl(Native Method)
E/FlashlightButton(  133):      at org.apache.harmony.luni.platform.OSFileSystem.open(OSFileSystem.java:152)
E/FlashlightButton(  133):      at java.io.FileInputStream.<init>(FileInputStream.java:82)
E/FlashlightButton(  133):      at java.io.FileInputStream.<init>(FileInputStream.java:134)
E/FlashlightButton(  133):      at com.android.server.status.widget.FlashlightButton.getFlashlightEnabled(FlashlightButton.java:73)
E/FlashlightButton(  133):      at com.android.server.status.widget.FlashlightButton.updateState(FlashlightButton.java:47)
E/FlashlightButton(  133):      at com.android.server.status.StatusBarService.updateStates(StatusBarService.java:2043)
E/FlashlightButton(  133):      at com.android.server.status.StatusBarService.updateWidget(StatusBarService.java:2065)
E/FlashlightButton(  133):      at com.android.server.status.StatusBarService.access$800(StatusBarService.java:120)
E/FlashlightButton(  133):      at com.android.server.status.StatusBarService$7.onReceive(StatusBarService.java:2094)
E/FlashlightButton(  133):      at android.app.ActivityThread$PackageInfo$ReceiverDispatcher$Args.run(ActivityThread.java:892)
E/FlashlightButton(  133):      at android.os.Handler.handleCallback(Handler.java:587)
E/FlashlightButton(  133):      at android.os.Handler.dispatchMessage(Handler.java:92)
E/FlashlightButton(  133):      at android.os.Looper.loop(Looper.java:123)
E/FlashlightButton(  133):      at com.android.server.ServerThread.run(SystemServer.java:524)

This is beacause, all power buttons are instantiate, even if unused or
functionnality is missing. So, I remove most calls to getInstance() for
all of them and use a HashMap to store the buttons used and only call
them in updateStates() and updateWidget().

Additionnaly, my change reduce memory usage of the status bar.

Fixed tabs, some indents and long lines.

Change-Id: I22b98bddc791c547d02b50a09b561aa469cc25b7