Add metadata store to surfaces

This adds a key/value metadata storage mechanism to
surfaces that allows the windowmanager to pass information
to the surfaceflinger frontend.

This then moves the existing metadata (window type and
ownerUID) into this metadata structure.

Bug: 122925737
Test: Phone boots and surfaces fling. Some unittests
Change-Id: I72c574737b7f75be2311a341812b15d385f507ed