Query for BLAST flag outside of constructor

WindowManagerGlobal should query for flag only when the service has
successfully been retrieved.  Moving query into block where
sWindowManagerService is initalized

Bug: 149243163
Test: build, boot, adb shell device_config put
window_manager_native_boot wm_use_blast_adapter true/false

Change-Id: I864d60158236059fda9844844b07299bb97ee6d1