Make Paint/Shader/ColorFilter#getNativeInstance conditionally thread-safe

Bug: 159246162
Bug: 154358781
Test: PaintNativeInstanceTest.kt

Although the android.graphics package makes no promises regarding
thread-safety, it is not obvious to clients how they might call
getNativeInstance from multiple threads. If one thread sets up a Paint,
and then two threads use it, this sets up a race condition for setting
up the native SkPaint. This seems to have been observed in internal
apps. Put accesses and modifications to these native objects inside
a synchronized block, so that we don't end up with a situation where
two instances of mCleaner attempt to dereference the same native object.

Add tests in PaintNativeInstanceTest.kt

Change-Id: I10fa0958ea216bfbc79667c6217dd6570b3eb110