Add cross-platform <android-base/endian.h>.

Bionic has <sys/endian.h>, glibc <endian.h>, and macOS and Windows have
nothing. This has often been annoying.

Bug: N/A
Test: new tests
Change-Id: I2a40c570df6a9bb30607ace1af653265938cc4b8
diff --git a/base/Android.bp b/base/Android.bp
index 694e029..7b1dc8e 100644
--- a/base/Android.bp
+++ b/base/Android.bp
@@ -78,6 +78,7 @@
     host_supported: true,
     clang: true,
     srcs: [
+        "endian_test.cpp",
         "errors_test.cpp",
         "file_test.cpp",
         "logging_test.cpp",