Move install to separate module

Build libinstall as a shared library. Also drop the dependency on the
global variables in common.h.

Test: unit tests pass, sideload an OTA
Change-Id: I30a20047768ce00689fc0e7851c1c5d712a365a0

tests/unit/package_test.cpp

@@ -26,7 +26,7 @@
 #include <ziparchive/zip_writer.h>
 
 #include "common/test_constants.h"
-#include "package.h"
+#include "install/package.h"
 
 class PackageTest : public ::testing::Test {
  protected: