Free wrapper resource before assignment

Any resources owned by "this" (lefthand side of assignment) are never
released (as the destructor would do), but are overwritten by the
move-assignment process and are therefore leaked.

This CL frees the resource before assignment. Additionally, it adds
missing ownership documentation for Compilation.

Bug: 79205067
Test: mma
Test: NeuralNetworksTest_static
Change-Id: Iacd561adee5205cb039fc7dea4fd1a816ca8d641
Merged-In: Iacd561adee5205cb039fc7dea4fd1a816ca8d641
(cherry picked from commit aec086a4)