Fix API for C compatibility

In https://android-review.googlesource.com/c/platform/system/core/+/1224544
when we introduced a default argument to androidSetThreadPriority(),
we broke C compatibility with this API, and with the header file
in general.

We fix this up by instead introducing a new method that takes
three arguments.  This gets this header file compiling for C again,
and keeps this particular API C compatible.

Any C++ callers of the three argument version of
androidSetThreadPriority() will need to switch to using
androidSetThreadPriorityAndPolicy().  Although since this was
a recent change, we believe there is only one such user, which
we are fixing at the same time.

Test: TreeHugger
Bug: 165009705
Merged-In: Iab0b7e6c91a8e32a17ba1b186fd0c2fe96b601e4
Change-Id: Iab0b7e6c91a8e32a17ba1b186fd0c2fe96b601e4