bionic-unit-tests: Build host testlibs in bionic-loader-test-libs

The target test libs are now all built in
nativetest/bionic-loader-test-libs. However, libs built using
libs/Android.build.testlib.mk are still put in lib/ on the host.

This commit refactors libs/Android.build.testlib*.mk so that all test
libs go into nativetest/bionic-loader-test-libs on the host; nothing
changes on the target. Android.build.mk now understands
*_install_to_native_tests_dir both for host and target builds.

Change-Id: I1022e9bab38bf53036c95fd39b3b240bcb31f23a