libui: Fix clamping of input values in ui::Size

A previous change to fix a compiler warning broke the clamped-conversion
of floating point input values to the int32_t values used internally.
As a result, calling setWidth() or steHeight() with any floating point
value resulted in a value of INT32_MIN being set,

The existing Size_test unit test would have caught this issue, but it
would have had to have been manually run since it was not configured to
run automatically. No one noticed it was broken for a while since it is
relatively new and the existing callers were not setting a floating
point value.

This change:

1) Fixes clamping of floating point values to work again, as verified
   by the existing test.

2) Helps enforce that the header remains conversion-warning free by
   enabling a few conversion warnings (as errors) when building the
   test.

3) Adds a TEST_MAPPING file, which will run Size_test as part of the
   presubmit runs (effectively continuously), as well as including the
   test in the device-test suite in the Android.bp file.

4) Adds a PrintTo() function for more clearly printing out a ui::Size
   value when using the Google Test framework.

Bug: 149495759
Test: atest Size_test
Change-Id: I8f18f20e6b5a3ea54eb383486a1f7222a1a2a544