Switch libplatformproto to sharedlib

libplatformprotos is really really unhappy if it gets
loaded twice into the same process which trivially happens
if it's a static library. Switch it to a shared library
instead to fix this.

Change-Id: I90297d076d0739bab683fbd9607d7fc0884a002d
Fixes: 36272398
Test: Ran hwui_unit_test, didn't crash