Rename servicetool to pdx_tool

As we are planning to include the pdx diagnosis binary into Android
system image, the name of servicetool is too general to fit in. Rename
it to pdx_tool as it's quite unique, short and sufficiently descriptive
to what it actually is.

Bug: 64802424
Test: Build pdx binary, push to device and dump
Change-Id: I9b5e9094e2253e994bd4b7ec9e47af5c0b442531