Add separate native library for instrumentationtests

Also adds api in PCF for specifying which library to load.

This is in preparation for a CL adding a native function
to be used only in tests.

Bug: webrtc:9225
Change-Id: I72eff272350404729424176758bfa81f7da81836
Reviewed-on: https://webrtc-review.googlesource.com/84125
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Paulina Hensman <phensman@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23663}

sdk/android/instrumentationtests/src/org/webrtc/HardwareVideoEncoderTest.java

@@ -354,7 +354,7 @@ public class HardwareVideoEncoderTest {
   // # Tests
   @Before
   public void setUp() {
-    NativeLibrary.initialize(new NativeLibrary.DefaultLoader());
+    NativeLibrary.initialize(new NativeLibrary.DefaultLoader(), TestConstants.NATIVE_LIBRARY);
 
     eglBase = new EglBase14(null, EglBase.CONFIG_PLAIN);
     eglBase.createDummyPbufferSurface();