On the Android platform prefer using the ASM code to retrieve
TLS context object even for single threaded apps. This helps
get rid of a branch in GetGlobalContext() and GetValidGlobalContext()
further improving the CPU perf of TLS operations.

Bug: angleproject:4717
Change-Id: I58d3d3b7061d613b24f945c07bed497c7c4be25c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2250318
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>