Avoid acquiring a lock on the InputReader thread

This is	a follow up CL to my previous CL [1], which might have
introduced a possible performance bottleneck because of holding a lock
on the InputReader thread.

With this CL addresses that concern by changing
PhoneWindowManager#mDismissImeOnBackKeyPressed to volatile that is long
used inside PhoneWindowManager.

 [1]: I20721547c73360a70b5fc5cbe06824d577d1768a
      d6475a68

Bug: 33824860
Test: Manually tested as follows
       1. Open dialer and focus in to the text field to show an IME
       2. Run 'adb shell dumpsys window policy' to make sure
            mDismissImeOnBackKeyPressed=true
       3. Tap the back button to dismiss the IME
       4. Run 'adb shell dumpsys window policy' to make sure
            mDismissImeOnBackKeyPressed=false
Change-Id: I0d0d06b0b848798c0f488b138812d932432b1d4d