Only position dialog vertically

Prior to change I78d8c78863bd6ff9c0b105595420e91b6b741754 the
GrantPermissionActivityDialog window was only positioned using the
vertical component of PermissionGrantSingleton. By forcing the window
postion to be centered horizontally we restore that logic

Test: Manually verified that dialog is positioned at bottom center when
PermissionGrantSingleton gravity is set to left|bottom
Bug: 207733469
Bug: 200801402

Change-Id: I07e9f2eec169149eebcfbf2b6f38bf69fefbd4b3