Use mMergedConfig instead of mGlobalConfig while getting winConfigFromWm

Multiple activities in the same process might share the same global
config. In such cases, the global config might NOT describe the bounds
of a window which receives the config. This CL assigns
windowConfiguration from mMergedConfig to winConfigFromWm.

Fix: 360905410
Fix: 298433614
Flag: EXEMPT bugfix
Test: 1. Open 2 instances of Chrome in split-screen mode by selecting
         "New window" from it menu on a large screen device.
      2. Make sure the bounds of winConfigFromWm match the real bounds
         of each instance of Chrome respectively.
Change-Id: Ic444bd400929531f4b8a9d8966bda8ca9f170057