Atrace:use ReadFileToString to read file

Since wildcard support was added to atrace, it can happen
very easily that file size is large than 4096
resulting in bad status that atrace cannot find kernel function
This patch use android::base::ReadFileToString to read file
which doesn't have size limitation.

Signed-off-by: Stephane Gasparini <stephane.gasparini@intel.com>
Change-Id: Ic24a9267053302a03ff04c8b2afeb143e5b94c84
Signed-off-by: Zhiquan Liu <zhiquan.liu@intel.com>