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/asn1_decoder_test.cpp

@@ -20,7 +20,7 @@
 
 #include <gtest/gtest.h>
 
-#include "asn1_decoder.h"
+#include "private/asn1_decoder.h"
 
 TEST(Asn1DecoderTest, Empty_Failure) {
   uint8_t empty[] = {};