Add PreloadPublicNativeLibraries function

Shift responsibility of preloading native libraries to the user
of the library. This lets applications that do not use jni but
depend on libnative_loader.so to avoid unnecessary preloads of
public libraries which could be expensive.

Bug: http://b/27245894
Change-Id: I12a8100de5da639c068769bca21ecebff2b2d538