1. Use contextId as identifier to separate captures for different GL
context.
2. Fix write string params. In order to keep consistent between replay
and capture, changes have been made to store C style string (with '\0'
suffix) for string param capture, which breaks cpp frame capture. This
change fixes it.
3. On Android device, it use a custom path to save captures and it
breaks the loadBinaryData(). Here differentiate file name and file path
to fix it.

Bug: angleproject:3804
Change-Id: I378cf26697d15f0c1a8cf7afea2b758f5c58498b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1774469
Commit-Queue: Jiacheng Lu <lujc@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>