Introduce multi-frame SKP capturing in SkiaPipeline

Capture script usage is the same as before, but all frames are combined
into a single file which shares the images between frames, reducing the
file size and serialization time somewhat. This brings us closer to the
objective of realistic performance measurements, but to do that
correctly, a second format is needed, (skbug.com/9174)

Single frame captures still produce the same format.

Test: The method used for serialization is tested in skia's DM unit
tests, in MultiSkpTest.cpp
Test: Tested manually with the libs/hwui/tests/scripts/skp-capture.sh script
Bug: skbug.com/9210
Change-Id: I69da8d191640ebb444991f107d60389f1519a9db