Enable ThinLTO for hwui

ThinLTO will benefit hwui on both performance and code size.

Now that hwui is with PGO, ThinLTO provides 3.8% performance improvement
on top of it (tested using hwui benchmarks on marlin device).

Enabling ThinLTO also decreases current code size of libhwui.so by 3.2%.

To be mentioned, Thinlto property in bp file will go through all static
libraries that hwui depends on (such as libskia) and enable Thinlto for
them when building hwui.

Test: Build and run benchmarks with ThinLTO enabled.
Bug: http://b/62839002
Change-Id: I89466d50c3715f6e7b0518ff10240bf8a5e9714c