Properly detect the presence of TA

Properly detect the presence of TA

Tef-simulator and optee use different TA file name formats. Key-manager was
detecting the presence of TA by checking the existence of TA file with
hardcoded format. It worked with tef-simulator but it failed to detect the TA
presence in case of optee.

This commit replaces the TA file presence checking with an attempt to open a
session using libteec. If an attempt succeeds the decider selects TZ backend.
Otherwise, it falls back to SW backend.

Change-Id: I840d6b58a1ffa39885a4b8ded0ff70f4147c3de0