Soong: Add mk2bp conversion for prebuilt_font.

There are a couple of makefiles that have etc with $(TARGET_OUT)/fonts
and $(TARGET_OUT_PRODUCT)/fonts  as the local module path. Added the
conversion in androidmk for target.

bug: b/123403785
Test: Wrote and ran unit test cases, did a test conversion on
        * platform/external/google-fonts/dancing-script/Android.mk
        * internal/external/google-fonts/arbutus-fonts/Android.mk
Change-Id: Id67e10f80b32b27c5027cd274a7944801dad0880