Some refactoring of StaticLayout

This patch refactors construction of StaticLayout to use an explicit
Builder object, which is intended to hold state used for constructing
the layout but not needed for merely reading out the results.

Builder objects are allocated from a pool and explicitly recycled,
so there is insignificant additional allocation cost.

This patch has very little impact on performance (it does avoid
allocating a FontMetricsInt object) but opens the way for significant
performance and functionality improvements as more of the Builder
functionality migrates to native code.

Change-Id: I2a576643e573a38b61f895a80d5d92a85c94b6b4