ueventd: Wait for runtime apex before running external firmware handler

External firmware handlers cannot run until com.android.runtime.apex is
activated. However, it may be possible to request firmware from the
kernel before apex activation. Waiting for runtime apex is required
before running an external firmware handler.

Test: atest CtsInitTestCases
Change-Id: Id63b7f29084d3cecb901049162d3e5cd0055566f
Signed-off-by: Suchang Woo <suchang.woo@samsung.com>