gralloc: Adds a new inline method map_usage_to_memtrack.

gralloc: Adds a new inline method map_usage_to_memtrack.

map_usage_to_memtrack takes in the gralloc usage flag and returns a
const char pointer containing the memtrack tag.

This function should be used to find the appropriate memtrack tag that
needs to be used after allocating buffers. This will help in-kernel
memtrack module to classify memory into pre-defined buckets.

Bug: 27525688
Change-Id: Ieee868f9b90ab170c16ba80aceedcb3b2e3762df
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>