Don't dlclose libperfetto_android_internal.so once loaded

Prior to this CL:
1) the internal .so was lazily loaded the first time a process
   needed to access a non-NDK function.
2) It was unloaded once done

2 turns out to be particularly problematic because causes all
sorts of FD leaks. Most libraries don't handle dlcose() proprely.
This CL cleans up a bit the dlopn mechanism, centralizing it in
one place and never dlcloses the opened library once loaded.


Test: manual (using android.power)
Bug: 137280403
Change-Id: I21006ebafb35a063c0ca5c9c2d370c049fb27eb8