Add unittests for file_scanner. Fix minor bug.

Bug: 73625480
Change-Id: Ice90c62ee6dd9742add9627cf7d8c9b9f2e6d741

Android.bp

diff --git a/Android.bp b/Android.bp
index ad89c9d..46ce586 100644
--- a/Android.bp
+++ b/Android.bp
@@ -3374,6 +3374,7 @@
     "src/protozero/test/fake_scattered_buffer.cc",
     "src/protozero/test/protozero_conformance_unittest.cc",
     "src/traced/probes/filesystem/file_scanner.cc",
+    "src/traced/probes/filesystem/file_scanner_unittest.cc",
     "src/traced/probes/filesystem/fs_mount.cc",
     "src/traced/probes/filesystem/fs_mount_unittest.cc",
     "src/traced/probes/filesystem/inode_file_data_source.cc",