Add a new module named prebuilt_dsp.

prebuilt_dsp soong module allows to install DSP related file to
<partition>/etc/dsp directory. If soc_specific property is enabled
to true, it is then installed to <partition>/dsp directory for
vendor image.

Bug: b/157259542
Test: Wrote unit test cases.
Change-Id: I15431a14bf399338a00835718dfe29544be02e34