Fix Sharesheet grid items not aligned issue

The "layout_width" field was set to fixed dp for the top level
linearlayout of grid item. Changing it to "match_parent" can fix the
issue. The location of inner layouts are centered now.

Bug: 148515684
Test: build and flashed to crosshatch device. Grid items in sharesheet
are aligned.

Change-Id: Id4ab6c44bfadf7caa00b856d3dba6dbb791200b5