This PR reduces the number of measure calls for the children of the
Flexbox.
If any items in a FlexLine don't have the flexGrow or flexShrink
attributes set, the children in the FlexLine don't have to be measured
in the FlexboxHelper#determineMainSize method.

Similarly, if the AlignItems attribute for the Flexbox is not equal to
AlignItems.STRETCH nor any items in a FlexLine don't have the
AlignSelf.STRETCH attributes, the items in the FlexLine don't have to be
measured in the FlexboxHelpers#stretchViews method.

Also this PR changes the default values of FlexboxLayout as follows:
- mAlignItems: STRETCH -> FLEX_START
- mAlignContent: STRETCH -> FLEX_START