libgui: Fix naming/enums in ISurfaceComposerClient

Moves the Tag enum out of the ISurfaceComposerClient header and into an
anonymous namespace in the .cpp file, changes it to UPPER_UNDERSCORE
naming, and makes its LAST entry actually point to the last value.

Test: m -j + manual testing
Change-Id: I73deaa446a87709548c2069c3a1f29605967ab97