style: Add fixmes for a few issues the RN/Flow upgrade turns up.

Without this change, after the upcoming upgrades Flow gives error
messages here.  If I understand the error messages correctly (though
I'm not confident I do), the point is that these particular components
insist on their props being read-only -- and while ViewStyleProp and
friends make that guarantee, DangerouslyImpreciseStyleProp doesn't.

These will be resolved properly when we eventually go about and narrow
down uses of DangerouslyImpreciseStyleProp to the more specific types.
Until then, just leave these fixmes.

src/common/KeyboardAvoider.js

@@ -24,7 +24,7 @@ export default class KeyboardAvoider extends PureComponent<Props> {
 
     return (
       <KeyboardAvoidingView
-        behavior={behavior}
+        behavior={behavior /* v--- $FlowFixMe wants ViewStyleProp */}
         contentContainerStyle={contentContainerStyle}
         style={style}
       >

src/common/Screen.js

@@ -112,7 +112,10 @@ class Screen extends PureComponent<Props> {
         >
           {scrollableContent ? (
             <ScrollView
-              contentContainerStyle={[centerContent && componentStyles.content, style]}
+              contentContainerStyle={
+                /* $FlowFixMe wants ViewStyleProp */
+                [centerContent && componentStyles.content, style]
+              }
               style={componentStyles.childrenWrapper}
               keyboardShouldPersistTaps={keyboardShouldPersistTaps}
             >

src/common/Touchable.js

@@ -49,7 +49,7 @@ export default class Touchable extends PureComponent<Props> {
       return (
         <TouchableHighlight
           accessibilityLabel={accessibilityLabel}
-          underlayColor={HIGHLIGHT_COLOR}
+          underlayColor={HIGHLIGHT_COLOR /* v--- $FlowFixMe wants ViewStyleProp */}
           style={style}
           onPress={onPress}
           onLongPress={onLongPress}