Move Frame Events to a separate table and add frame stats table

Since the requirements of frame events are changing periodically, tying
them to gpu_slice proves to be restrictive in the ways of adding new
info and collecting stats.

This change addresses this concern by
creating a separate table for frame events while also adding a new
frame_stats table.

Test: diff_test_trace_processor.py --trace-filter graphics_frame_events
Bug: 152533807

Change-Id: I2c1e41096605df9789089cb611345592a9a0a5d4