Size modifier API changes

Change size layout modifier API organization to the following
to aid in discoverability and grouping:

LayoutWidth - exact size modifier type; grouping for other width
constraints

LayoutHeight - exact size modifier type; grouping for other height
constraints

LayoutSize - exact size modifier type; grouping for other width+height
constraints

This reduces the number of related symbols at the top level and permits
easier discovery of other things you can do with width, height and size
through dot-based autocomplete.

Removes top-level factory functions using PascalCase that do not return
an object of the type named by the function.

Bug: 145696525
Test: existing layout tests

Change-Id: I9fa86f6205ab025135b621c3219e4d6f3d05e77f

diff --git a/ui/ui-material/src/main/java/androidx/ui/material/Snackbar.kt b/ui/ui-material/src/main/java/androidx/ui/material/Snackbar.kt
index c97fccd..56084b2 100644
--- a/ui/ui-material/src/main/java/androidx/ui/material/Snackbar.kt
+++ b/ui/ui-material/src/main/java/androidx/ui/material/Snackbar.kt

@@ -37,9 +37,9 @@
 import androidx.ui.layout.AlignmentLineOffset
 import androidx.ui.layout.Column
 import androidx.ui.layout.Container
-import androidx.ui.layout.LayoutExpandedWidth
 import androidx.ui.layout.LayoutGravity
 import androidx.ui.layout.LayoutPadding
+import androidx.ui.layout.LayoutWidth
 import androidx.ui.material.surface.Surface
 
 /**
@@ -170,7 +170,7 @@
     button: @Composable() () -> Unit
 ) {
     Column(
-        modifier = LayoutExpandedWidth + LayoutPadding(
+        modifier = LayoutWidth.Fill + LayoutPadding(
             left = HorizontalSpacing,
             right = HorizontalSpacingButtonSide,
             bottom = SeparateButtonExtraY