EmbeddedPkg/AndroidFastboot: eliminate deprecated string function calls
Get rid of calls to unsafe string functions. These are deprecated and may
be removed in the future.
Note that this also addresses a latent potential issue in HandleDownload(),
where NumBytesString[] (which comes from the wire) is assumed to contain a
string representation of a number with all the significant digits in the
first 8 bytes, which is not guaranteed by the protocol.