Improve Android support

This patch remove many Android-specific #ifdefs from the Breakpad
source code. This is achieved by providing "fixed-up" platform
headers (e.g. <signal.h> or <sys/user,h>), in the new directory
src/common/android/include/, which masks differences between
the NDK and GLibc headers.

The old "android_link.h" and "android_ucontext.h" are moved
and renamed.

This also requires putting this directory as the first
include path during Android-hosted builds, hence the
modification of Makefile.am and configure.ac
Review URL: https://breakpad.appspot.com/434002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1017 4c0a9323-5329-0410-9bdc-e9ce6186880e

src/client/linux/handler/exception_handler.cc

@@ -77,12 +77,10 @@
 #include <sys/wait.h>
 #include <unistd.h>
 
-#if !defined(__ANDROID__)
 #include <sys/signal.h>
 #include <sys/ucontext.h>
 #include <sys/user.h>
 #include <ucontext.h>
-#endif
 
 #include <algorithm>
 #include <utility>