Move adb_dirname and adb_basename to libbase

adb already provides an implementation of dirname and basename that
take and produce std::strings, move it into libbase so it can be
used in the implementation of GetExecutableDirectory.

Test: out/host/linux-x86/nativetest64/adb_test/adb_test
Test: out/host/linux-x86/nativetest64/libbase_test/libbase_test
Test: adb shell /data/nativetest64/libbase_test/libbase_test64
Change-Id: Ideb1627607b14562121316d4ed27fa6fb0930684