Add android::base::GetExecutablePath, switch adb and fastboot over.

We'd long had two copies of this stuff, so rather than rewrite both
Linux versions to use android::base::Readlink, let's kill the duplication
too...

Bug: http://b/30988271
Change-Id: I4de58a94a22a4b1faf969a6fc70ca1560a4d5121
diff --git a/base/file_test.cpp b/base/file_test.cpp
index ca01ee8..f5d6062 100644
--- a/base/file_test.cpp
+++ b/base/file_test.cpp
@@ -136,3 +136,7 @@
   ASSERT_EQ(max, result);
 #endif
 }
+
+TEST(file, GetExecutablePath) {
+  ASSERT_NE("", android::base::GetExecutablePath());
+}