Set AlertDialog width to M specs

Honor the material specs:
- Width must be at least 280dp and at most 560dp
- Default width should be 312dp for parity with Android
- Margins should be at least 24dp

Note that the margins are enforced by the dialog window on Android, so
they don't have to be added in the Compose part.

Fixes: 144798445
Test: ./gradlew ui:ui-material:cC
Change-Id: Ibd506bf9d72695454bb92b2ec831bceef1b3d1ed

diff --git a/ui/ui-foundation/src/main/java/androidx/ui/foundation/Dialog.kt b/ui/ui-foundation/src/main/java/androidx/ui/foundation/Dialog.kt
index 3c37f3d..649e721 100644
--- a/ui/ui-foundation/src/main/java/androidx/ui/foundation/Dialog.kt
+++ b/ui/ui-foundation/src/main/java/androidx/ui/foundation/Dialog.kt

@@ -72,6 +72,8 @@
 
     onCommit {
         dialog.setContent {
+            // TODO(b/159900354): draw a scrim and add margins around the Compose Dialog, and
+            //  consume clicks so they can't pass through to the underlying UI
             Box(Modifier.semantics { this.dialog = true }, children = children)
         }
     }
@@ -94,6 +96,8 @@
         ViewTreeViewModelStoreOwner.set(frameLayout, ViewTreeViewModelStoreOwner.get(composeView))
     }
 
+    // TODO(b/159900354): Make the Android Dialog full screen and the scrim fully transparent
+
     fun setContent(children: @Composable () -> Unit) {
         // TODO: This should probably create a child composition of the original
         composition = frameLayout.setContent(recomposer, children)

diff --git a/ui/ui-material/src/main/java/androidx/ui/material/AlertDialog.kt b/ui/ui-material/src/main/java/androidx/ui/material/AlertDialog.kt
index ef38f3b..add9ccd 100644
--- a/ui/ui-material/src/main/java/androidx/ui/material/AlertDialog.kt
+++ b/ui/ui-material/src/main/java/androidx/ui/material/AlertDialog.kt

@@ -194,7 +194,7 @@
     }
 }
 
-private val AlertDialogWidth = Modifier.preferredWidth(280.dp)
+private val AlertDialogWidth = Modifier.preferredWidth(312.dp)
 private val ButtonsBoxModifier = Modifier.fillMaxWidth().padding(all = 8.dp)
 private val ButtonsWidthSpace = Modifier.preferredWidth(8.dp)
 private val ButtonsHeightSpace = Modifier.preferredHeight(12.dp)
@@ -205,4 +205,4 @@
 private val TitlePadding = Modifier.padding(start = 24.dp, top = 24.dp, end = 24.dp, bottom = 0.dp)
 // The height difference of the padding between a Dialog with a title and one without a title
 private val NoTitleExtraHeight = Modifier.preferredHeight(2.dp)
-private val TextToButtonsHeight = Modifier.preferredHeight(28.dp)
\ No newline at end of file
+private val TextToButtonsHeight = Modifier.preferredHeight(28.dp)