Remove "exec_type" from postinstall_file.

update_engine had an automatic transition to the "postinstall" domain
when executing a "postinstall_file" which required it to be an
entrypoint. This patch removes this automatic transition and the
associated rules in update_engine.te, removing as well the need to
add exec_type to postinstall_file. Instead, update_engine now makes
this transition explicit by calling setexeccon(3).

Bug: 28008031
TEST=make dist; Deployed an update to edison-eng: postinstall runs as "postinstall" domain.

Change-Id: I2b799ac4808c90b010a9e776aaa7015020a94b49