frameworks/base: Fix to avoid UI freeze issue.

Due to some race condition in WindowManagerService, Screenshot
layer remains as topmost and buffers are not flipped hence
no updates seen on the screen. So, trying to destroy persistent
layer explicitly if freeze timeout expires 6sec.

Change-Id: Ibe295550a7431a4f633a57e7dfab0cbda102ab87
Merge "frameworks/base: Fix to avoid UI freeze issue." into lollipop