Refactor libprofile-extras to be added as a whole static library

Bug: http://b/134177005
Bug: http://b/116873221

Previously, the libprofile-extras dependency was added as a
LateStaticLib and the constructor in this library was included during
linking with the '-uinit_profile_extras' linker flag.  This was done
because at the deps() stage, the exact binaries that need coverage are
not known (in fact the coverage-enabled variants are not created yet).

This meant that for a link command, if one of the shared libraries
already exported the constructor, the output of the link command did not
load/link libprofile-extras.

For other reasons, we now want to add more symbols to this library that
need to be linked into all libraries and executables.  To accomplish
that, refactor the dependency handling so libprofile-extras can be added
as a 'WholeStaticLib'.

This is done by creating a new dependency type (with a coverageDepTag
dependency tag) to add libprofile-extras as a dependency for all modules
that can potentially link with coverage.  During the flags() call, this
dependency is moved as a WholeStaticLib dependency iff coverage is
enabled in this link step.

There are a few NFC changes as well:
- deps() takes a DepsContext parameter.
- flags() has an extra PathDeps parameter and return value.
- add useSdk() helper to cc.Module.

Test: Build with coverage and check that we can generate coverage using
SIGUSR1 and the debug.coverage.flush sysprop.

Change-Id: I7e7d8201956a150febbda5bb1794f8ece016db8b