Update Android.mk finder to work with Android.bp files

The Android.mk finder should stop traversing when it finds an Android.bp
file, and any Android.soong.mk file at the same path should be included.

Update filter-soong-makefiles to accept a mixed list of Android.bp files
and Android.mk files, convert Android.bp files to Android.soong.mk files
if they exist, and ignore Android.mk files that have an Android.bp file.

Update first-makefiles-under to use the new --dir syntax for
findleaves.py and search for both Android.bp and Android.mk files.

Update all-makefiles-under and all-named-subdir-makefiles to search for
both Android.bp and Android.mk files.

Change-Id: Iddc51bf84593d5de3add02f2b2c065da08962af4