fix(select,input): inconsistent disabled text color

* Fixes the select and input having different disabled text colors since the color isn't based on a palette. Adds a mixin to ensure that they can't get out of sync in the future.
* Fixes the input having the wrong disabled text color (0.38 alpha for light themes and 0.5 for dark ones). It seems like the colors that were used before were in reference to the underline, not the text (see https://material.io/guidelines/components/text-fields.html#text-fields-states).

Fixes angular#7793.

src/lib/core/style/_form-common.scss

@@ -6,3 +6,11 @@
   background-size: 4px 1px;
   background-repeat: repeat-x;
 }
+
+// Determines the text color of a disabled control since the values are not part of a palette.
+@mixin mat-control-disabled-text($theme) {
+  $foreground: map-get($theme, foreground);
+  $is-dark-theme: map-get($theme, is-dark);
+
+  color: mat-color($foreground, secondary-text, if($is-dark-theme, 0.5, 0.38));
+}

src/lib/input/_input-theme.scss

@@ -5,11 +5,8 @@
 
 
 @mixin mat-input-theme($theme) {
-  $foreground: map-get($theme, foreground);
-  $is-dark-theme: map-get($theme, is-dark);
-
   .mat-input-element:disabled {
-    color: mat-color($foreground, secondary-text, if($is-dark-theme, 0.7, 0.42));
+    @include mat-control-disabled-text($theme);
   }
 }
 

src/lib/select/_select-theme.scss

@@ -10,12 +10,6 @@
   $primary: map-get($theme, primary);
   $accent: map-get($theme, accent);
   $warn: map-get($theme, warn);
-  $is-dark-theme: map-get($theme, is-dark);
-
-  .mat-select-disabled .mat-select-value,
-  .mat-select-arrow {
-    color: mat-color($foreground, secondary-text);
-  }
 
   .mat-select-content, .mat-select-panel-done-animating {
     background: mat-color($background, card);
@@ -25,6 +19,14 @@
     color: mat-color($foreground, text);
   }
 
+  .mat-select-disabled .mat-select-value {
+    @include mat-control-disabled-text($theme);
+  }
+
+  .mat-select-arrow {
+    color: mat-color($foreground, secondary-text);
+  }
+
   .mat-select-panel {
     .mat-option.mat-selected:not(.mat-option-multiple) {
       background: mat-color($background, hover, 0.12);
@@ -51,13 +53,9 @@
     }
 
     .mat-select.mat-select-disabled .mat-select-arrow {
-      color: mat-color($foreground, secondary-text);
+      @include mat-control-disabled-text($theme);
     }
   }
-
-  .mat-select.mat-select-disabled .mat-select-arrow {
-    color: mat-color($warn);
-  }
 }
 
 @mixin mat-select-typography($config) {